feat(controller): emit result-tagged cache-lookup counters (#241)

Replace the hit-only `cache_hit` counters with `result`-tagged
cache-lookup counters at every controller cache-lookup site, so a
per-layer hit rate (`hit / (hit + miss)`) is derivable for each cache.

Three counters, each emitted under its parent RPC op with a
`result=hit|miss` tag:

| metric | emitted from |
|---|---|
| `treehash_cache_lookup` | `get_target_graph`, `get_changed_targets` |
| `graph_cache_lookup` | `get_target_graph` |
| `compared_targets_cache_lookup` | `get_changed_targets` |

## Classification

The result is classified by the lookup error's **origin**: a not-found
is a genuine miss; an infra failure is *not* a cache miss (it's already
tracked by the failure metric) and is left out so it can't skew the hit
rate.

| site | not-found | infra error |
|---|---|---|
| treehash | miss | nothing (request fails) |
| graph | miss | nothing (request fails) |
| compared-targets | miss | nothing (recomputes) |

**Breaking:** the bare `controller.get_target_graph.cache_hit` and
`controller.get_changed_targets.cache_hit` counters are removed.

Stacked on `refactor/share-metrics-buckets` (#240) — review/merge that
first.

feat(repomanager): emit lease metrics for health and slot-wait latency (

#232)

## What

Instruments `RepoManager.Lease` with the `observability/metrics`
emitter, using the same Scope-in pattern as the orchestrator and
graphrunner.

- `lease.start` counter + result-tagged
(`success`/`failure`/`cancelled`) `lease.finish` histogram — pool
health.
- Per-step latency histograms: `ensure_origin_duration`,
`wait_slot_duration` (the worker-slot wait — the saturation signal),
`create_worker_duration`.

Repo-tagged; buckets are exponential 1ms..~1.3h. The example server
leaves the scope no-op (as the orchestrator/graphrunner are wired there
today); production wiring supplies a reporter-backed scope.

Also folds nil-scope handling into `metrics.New` (nil → no-op, no error)
and adds `Emitter.SubScope`, removing the repeated `if scope == nil` /
`if err != nil { Nop() }` boilerplate across all call sites
(repomanager, controller, graphrunner, orchestrator).

## Test

`make build` / `make test` / `make gazelle` pass.

refactor(cachekey): move internal/cachekey to core/cachekey to make i…

…t importable (#213)

The cache key helpers are used in orchestrator implementations, so users
implementing their own orchestrator cannot import cachekey because it's
under internal. Move the package to core/ to make it importable.

## Test Plan
<!-- How did you test this? Provide evidence (screenshots, logs, or
steps to reproduce). -->
CI

## Issue
<!-- 
Link the issue here. 
- Use 'Closes #123' if this is the final fix. 
- Use 'Part of #123' or just '#123' if the feature is still in progress.
-->

feat(mapper): add mapper to proto TangoError (#201)

Add mapper to convert core/errors TangoError into the proto TangoError
message returned by RPCs.

Stacked on #169

## Test Plan
<!-- How did you test this? Provide evidence (screenshots, logs, or
steps to reproduce). -->
unit test
## Issue
<!-- 
Link the issue here. 
- Use 'Closes #123' if this is the final fix. 
- Use 'Part of #123' or just '#123' if the feature is still in progress.
-->

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

classify getGraph errors and consolidate failure reasons (#125)

Consolidate shared failure reason constants (cancelled, unknown,
storage, validation, deadline_exceeded) into core/common so they can be
reused across controller, orchestrator, and external orchestrator
implementations.

Classify previously-unclassified error paths in getGraph:
- context cancellation during graph read → cancelled/user
- context cancellation during orchestrator call → cancelled/user
- graph blob NotFound after treehash hit → falls through to compute
- non-NotFound storage errors → graph_fetch/infra
- unclassified orchestrator errors → graph_fetch/infra

## Test Plan
<!-- How did you test this? Provide evidence (screenshots, logs, or
steps to reproduce). -->
unit tests

## Issue
<!-- 
Link the issue here. 
- Use 'Closes #123' if this is the final fix. 
- Use 'Part of #123' or just '#123' if the feature is still in progress.
-->

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Classify context-cancellation errors during graph comparison correctly (

#121)

Summary:
Intent:
- if context is cancelled during `compareTargetGraphs`, the err gets
classified as an infra failure, compare, but it should be user failure,
canceled.

Changes:
- Added ctx.Err() guard before the generic error path in
compareTargetGraphs,
- When ctx is cancelled, errors now return
failureReasonCancelled/ErrorTypeUser instead of failureReasonCompare

Test Plan:
- Ran `bazel build //controller/...` — clean build

Revert Plan:
Revert this PR via `git revert <commit_sha>`.

---

<sub>Generated by the 🪄
[pr-create](https://sg.uberinternal.com/code.uber.internal/uber-code/devexp-agent-marketplace/-/blob/claude-code/plugins/dev/uber-dev/skills/pr-create/SKILL.md)
skill in devexp-agent-marketplace</sub>

<!-- Provide a summary in the Title above. Example: "Feature: add user
authentication" -->

## Why?
<!-- Why is this change necessary? What is the motivation or
justification? -->

## What?
<!-- What specifically is changing? Provide context for the reviewer.
-->

## Test Plan
<!-- How did you test this? Provide evidence (screenshots, logs, or
steps to reproduce). -->

## Issue
<!-- 
Link the issue here. 
- Use 'Closes #123' if this is the final fix. 
- Use 'Part of #123' or just '#123' if the feature is still in progress.
-->

Seed rules whose own source files changed at distance 0 (#119)

A rule whose srcs include a changed source file is now promoted to a BFS
seed (distance 0) instead of appearing at distance 1 behind the
source-file node. This makes distance == 0 mean "this target's own
inputs changed" rather than requiring consumers to look through the
source-file indirection.

The fix adds hasChangedSourceFileDep(), called during Pass 2 seed
classification, which checks whether any of a rule's direct dependencies
is both changed and a source file.

Behavior change: downstream targets of such rules shift one hop closer
in the BFS, so a given max_distance will include a slightly larger set.
This is more correct since the semantic seed is the rule, not the file.

Test Plan:
Unit tested
- Updated TestCompareTargetGraphs_SourceFileDirectAndPropagation and
TestCompareTargetGraphs_HashOnlyChangePropagatesViaBFS to expect
distance 0 for rules owning changed source files.
- Added TestCompareTargetGraphs_SiblingRuleNotPromotedToSeed: verifies a
rule depending on a sibling rule (not its own source file) stays at
distance 1.

## Issue
<!-- 
Link the issue here. 
- Use 'Closes #123' if this is the final fix. 
- Use 'Part of #123' or just '#123' if the feature is still in progress.
-->

Detach cancellation in cache-write goroutine; log infra errors (#117)

The compared-targets cache-write goroutine in GetChangedTargets used
context.Background(), losing the request's tracing/identity baggage.
Switch to context.WithoutCancel(ctx) so the write survives client
disconnect while keeping request values.
Per-operation deadlines stay out of the controller — storage.Storage is
backend-agnostic and must not encode any one backend's I/O budget.
Backends that need a deadline (e.g. YARPC/terrablob, which rejects
deadline-less contexts with InvalidArgument: missing TTL) own that
responsibility in their Put/Get implementations.

readTreehash now takes a *zap.Logger and logs any non-NotFound storage
error. The silent swallow is what hid this bug — a genuine cache miss
stays quiet, but an infra failure disabling the cache is now visible.
The goroutine also warns when it skips because a treehash was empty.


## Test Plan
Unit tests
tested in Uber's internal env

## Issue
<!-- 
Link the issue here. 
- Use 'Closes #123' if this is the final fix. 
- Use 'Part of #123' or just '#123' if the feature is still in progress.
-->

[controller] Emit histogram metrics for total duration (#109)

Summary:
Intent:
- Add M3 histogram metric for `total_duration` on both
`GetChangedTargets` and `GetChangedTargetsAndEdges` RPCs in preparation
for uMonitor latency alerts
- M3 timers are being deprecated (EoL June 30, 2026); histograms are the
replacement for percentile-based alerting

Changes:
- Added package-level `_totalDurationBuckets` (linear, 10s–15min, 90
buckets) and `totalDurationBuckets` field on the controller struct
- Emitted `total_duration.histogram` alongside the existing
`total_duration` timer at all four RPC completion paths (cache hit +
compute path for each API)

Test Plan:
- Existing unit tests in `controller/getchangedtargets_test.go` and
`controller/getchangedtargetsandedges_test.go` pass (histogram emission
uses `tally.NoopScope`)

Revert Plan:
Revert this PR via `git revert <commit_sha>`.

Refs: BUILD-379
Issues: LINEAR-BUILD-379

[controller] Emit histogram metrics for total duration (#109)

Summary:
Intent:
- Add M3 histogram metric for `total_duration` on both
`GetChangedTargets` and `GetChangedTargetsAndEdges` RPCs in preparation
for uMonitor latency alerts
- M3 timers are being deprecated (EoL June 30, 2026); histograms are the
replacement for percentile-based alerting

Changes:
- Added package-level `_totalDurationBuckets` (linear, 10s–15min, 90
buckets) and `totalDurationBuckets` field on the controller struct
- Emitted `total_duration.histogram` alongside the existing
`total_duration` timer at all four RPC completion paths (cache hit +
compute path for each API)

Test Plan:
- Existing unit tests in `controller/getchangedtargets_test.go` and
`controller/getchangedtargetsandedges_test.go` pass (histogram emission
uses `tally.NoopScope`)

Revert Plan:
Revert this PR via `git revert <commit_sha>`.

Refs: BUILD-379
Issues: LINEAR-BUILD-379