Add information about 2 additional metrics

[mhenc]

Updated docs/observability/metrics.md to document the new native emission of Google GenAI metrics by ADK and the behavior when external instrumentation is present.

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[koverholt]

Thanks for the updates. I now see that #1686 is in a version of ADK Python that is not yet released, which was not clear in the original PR. We will need to un-publish that page and hold off on publishing it until a version of ADK is released with metrics support.

[koverholt]

Update: I opened #1713 to readd the metrics page when it's available in a release. Should the content in this PR be merged along with #1713, or is it dependent on other upstream changes?

From what I can tell, these two metrics are emitted by opentelemetry-instrumentation-google-genai, not by ADK itself.

[mhenc]

Sorry for not letting you know that metrics are not yet released.

No, this change still waits for the PR to be merged. I will let you know when it's ok to review this one.