Add opt-in go.memory.gc.pause.duration histogram metric

[dashpole]

Part of #3353

Changes

Add an opt-in go.memory.gc.pause.duration histogram metric. This can be useful to see how long GC is taking, but the Go maintainers suggested that the existing go.schedule.duration is a better indicator of problems due to the program not being run -- hence why it is opt-in.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• Links to the prototypes or existing instrumentations (when adding or changing conventions)
  • open-telemetry/opentelemetry-go-contrib@main...dashpole:opentelemetry-go-contrib:optin_runtime_metrics

Gemini helped me split this from #3620

@open-telemetry/go-approvers @jmacd