feat: emit OTel gen_ai.client.* metrics natively

gen_ai.client.operation.duration and gen_ai.client.token.usage already
exist in OTel semconv
(https://opentelemetry.io/docs/specs/semconv/gen-ai/gen-ai-metrics/),
and are currently emitted by
https://github.com/open-telemetry/opentelemetry-python-contrib/tree/main/instrumentation-genai/opentelemetry-instrumentation-google-genai,
if configured. However, they have no agentic attributes.

This change builds on top of the recently introduced metric
instrumentation and emits those two metrics a new attribute:
gen_ai.agent.name. In the future, gen_ai.agent.version will also be set.

Semconv updates will be handled separately.

When the google-genai instrumentation lib is active for a Gemini agent,
we don't re-emit the gen_ai.client.* metrics to avoid double-counting.

Change-Id: Ie610d25ca44671d3a31d4307ebf068a1c9deeaab