Cherry Pick 8179 and 8002 into 2.1.9 (#1535)

Add GPU tests for all distros on a single GPU model (#1378)

Adds _aarch64 to DISTRO for kokoro sles15 presubmit (#1453)

Update Go build version to `1.20.8`. (#1441)

* Update Go build version to `1.20.8`.

* Follow `dev-docs/upgrade-go.md` for Go update.

Wait for fully running services before stopping (#1408)

This should prevent the Stop-Service failure flake we've been seeing.

Upgrade to Go 1.21.1 (#1407)

* Upgrade to Go 1.21.0

This PR updates the Ops Agent to Go 1.21.0. It also adds a new export
from the `internal/version` package to specify a Go version, which we
will now use in a couple of places that we need to know the Go version.

* not going to update this Go for now

* Go 1.21.1 literally just came out now lol

* dockerfiles: use ARG instead of templating

Rebase GPU preview branch (#1330)

Unenforce the `otlp_receiver` feature flag (#1368)

prometheus: support untyped metrics (#1312)

* prometheus: add config option to preserve untyped metrics

* prometheus: wire together the config flag for untyped metrics

* integration_test: refactor prometheus tests and test for untyped metrics

* prometheus: simplify config structure

* integration test: add test for reset points in untyped metrics

* integration test: add check to make sure the gauge-casted metric isn't reported when unknown metrics are preserved

* prometheus: rename config field for untyped metrics knob

Signed-off-by: Ridwan Sharif <ridwanmsharif@google.com>

Disable GPG checks when installing MySQL on SLES 12. (#1346)