Add health metrics for SDK metrics part

[JonasKunz]

Adds health metrics for the SDK metric parts. Based on the span health metrics from #1631 and log health metrics from #1921.

Changes

• Adds a new metric for the number of metric collections per metric-reader
• Adds exporter metrics for metric-exporters: these are pretty much a copy-paste, which is in accordance with Reality-check if exporter health metrics should be merged across signals #2002.

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]
• schema-next.yaml updated with changes to existing conventions.