docs: expose API metrics members in Sphinx

[iacker]

Summary

• add Sphinx member directives to docs/api/metrics.rst
• expose the opentelemetry.metrics API members on the generated docs page
• keep the change narrowly scoped to the smaller remaining docs gap from Some documentation not built into sphinx docs #2573

Validation

• python3 -m tox -e docs
  • build now reaches and writes api/metrics.html, and the generated page contains API symbols such as Counter, Histogram, Meter, NoOpMeter, and ObservableCounter
  • the full docs job still fails in this environment on a pre-existing Sphinx warning: <unknown>:1:py:class reference target not found: Token

Refs #2573

[xrmx]

Tried this but tox -e docs output does not change:

diff -ruN docs/oldbuild/html/ docs/_build/html/ | diffstat
 0 files changed