Add opt-in go.memory.gc.cycles counter metric

[dashpole]

Part of #3353

Changes

Add an opt-in go.memory.gc.cycles counter metric. This can be useful to see if the garbage collector is running more frequently than it should. I've kept it opt-in, since it wasn't part of the list of basic metrics recommended by the go team.

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