feat: add metrics backwards-compatibility CI framework

[Amaan729]

Closes #6278

Summary

Implements the first incremental slice of the metrics backwards-compatibility framework requested in #6278.

What this PR adds

.github/actions/verify-metrics-snapshot/ — new composite action

A reusable GitHub Actions composite action that:

1. Waits for Jaeger to start (retry loop), then curls the /metrics endpoint.
2. Uploads the raw Prometheus text as a GitHub artifact (keyed per storage backend, retained 30 days).
3. Diffs the live metric names against a committed baseline snapshot and fails CI if any metric was removed or renamed (additions are allowed and logged as a notice).

The action is already referenced in ci-e2e-memory.yaml via:

- uses: ./.github/actions/verify-metrics-snapshot
  with:
    snapshot: metrics_snapshot_memory
    artifact_key: metrics_snapshot_memory

testdata/metrics/ — baseline snapshot directory

Empty directory with a README explaining the workflow. Baseline .txt files are committed here after the first successful CI run.

What's intentionally out of scope for this PR

• Wiring the action into the remaining e2e workflows (cassandra, ES, OS, …) — follow-up PR.
• Downloading the previous release's artifact and diffing against it — follow-up PR once baselines are established.
• A Go script as an alternative scraper — the curl-based approach is simpler and has no build-time dependency.

How to establish the first baseline

After this PR is merged and a workflow run completes:

# Download the artifact from the Actions UI, then:
cp metrics_snapshot_memory.txt testdata/metrics/metrics_snapshot_memory.txt
git add testdata/metrics/metrics_snapshot_memory.txt
git commit -m "chore: add initial metrics snapshot baseline for memory backend"

Testing

• Action YAML syntax validated locally with actionlint.
• Logic tested by running the shell snippet manually against a local Jaeger instance.

Checklist

• Added composite action
• Added baseline directory + README
• Incremental scope (does not try to do everything in one PR)
• Baseline .txt files (will be committed in a follow-up after first CI run)