tempodb/blocklist: mark poller spans errored/ok - #7996

…giene)

Every span in the Poller.Do call chain (Poller.Do, Poller.Do.func,
Poller.pollTenantAndCreateIndex, Poller.pollTenantBlocks, pollBlock)
could return a non-nil error but never called SetStatus/RecordError,
so the span stayed Unset regardless of outcome - indistinguishable
from "nobody checked." pollUnknown was the one exception, but even
it never set Ok explicitly on its success path.

Add pkg/util/tracing.RecordErr and use it from a defer right after
each span is created, closing over each function's named (mostly
blank-named, `_`) return values so the existing return statements
don't need to change. Considered but did not collapse pollUnknown's
per-block span-in-a-loop (Poller.pollBlock): every storage backend's
Read/ListBlocks/etc. starts its own nested span from that per-block
ctx, so collapsing it would reparent those spans onto pollUnknown
instead, which is a real trace-structure change, not "just tracing."

Also switches the blocklist package's span tests off the swap-the-
global-TracerProvider-per-test pattern: OTel's global tracer only
delegates reliably to the *first* concrete provider installed via
otel.SetTracerProvider in a process, so a second span test doing the
same swap silently stopped capturing spans. Tests now share one
provider (installed once) and Reset() its exporter per test instead.