fix(export): add support in onGCE + no conn to metadata case

[bwplotka]

Fixes b/344740239 (edge case with GKE Metadata Server and GKE sandbox).

• Added debug logging.
• Updated metadata deps to get https://github.com/googleapis/google-cloud-go/pull/9733 & use timeout-ed context
• Moved risky logic from FromFlags, see code comment why.
• Added regession test.

Regression test fails on prev flow (no context propagation)

Alternatives

Everything we do in FromFlags or constructor is within readines period. We could consider moving potentially "slow" things on slow network or metadata srv to exporter.Run. This could be questionable as for GMP to work we at end need export functionality to work, so delaying that information or making it surface in separation to readiness might not be helpful.

EDIT: Added PR against metadata so we can get rid of custom code in the future: https://github.com/googleapis/google-cloud-go/pull/10370

[bwplotka]

I tested this PR on my repro cluster.

• std node: works
• sandbox node: works
• no metadata node: works (we pass data via flags so it's fine)
• sandbox, no metadata: collector runs, but export is unauth as expected

Metrics flow for 3 nodes as expected:

From this PR POV it's rdy to go I think

[bwplotka]

After I did https://cloud.google.com/stackdriver/docs/managed-prometheus/setup-managed#explicit-credentials all works:

[bwplotka]

🤗