feat(kagent-core): wire a MeterProvider so google-adk GenAI metrics export (gated OTEL_METRICS_ENABLED)

[MartinForReal]

Closes #2458

What

Wires a real MeterProvider into the Python agent runtime (kagent.core.tracing.configure()) so the seven GenAI metric instruments that google-adk already defines and records are exported instead of silently discarded.

Gated default-OFF behind OTEL_METRICS_ENABLED=true, matching the existing OTEL_TRACING_ENABLED / OTEL_LOGGING_ENABLED gates. When the gate is unset the runtime is byte-identical to today.

• python/packages/kagent-core/src/kagent/core/tracing/_utils.py — new _create_metric_exporter() mirroring the trace/log exporters (_resolve_otlp_protocol("METRICS"), endpoint, _resolve_otlp_timeout_seconds("METRICS")); builds PeriodicExportingMetricReader + MeterProvider(resource=...) and calls metrics.set_meter_provider(...). force_flush() now also flushes the metric reader, reusing the existing KAGENT_PRE_RESPONSE_TRACE_FLUSH pre-response flush to guard against the Agent Substrate checkpoint hazard for periodic readers.
• helm/kagent/values.yaml — new otel.metrics block alongside otel.tracing / otel.logging.
• tests/test_tracing_configure.py — 8 tests (default-OFF, env gating incl. garbage values, provider construction, signal-specific endpoint resolution, flush behavior).

Notes

• Verified google-adk (2.8.0) ships all seven instruments under meter scope gcp.vertex.agent and that token accounting incl. cached_content_token_count is handled upstream; ADK's own provider setup is not invoked by the kagent runtime, so the pipeline was genuinely absent.
• No new instruments and no new recording call sites.

Follow-up (not in this PR)

The Go-side env plumbing (OTEL_METRICS_ENABLED add to the env-forward allowlist so the flag reaches agent pods) is the companion work; see #2148/#2673. This PR is the Python/collection half.

Signed-off-by / generated by MartinForReal.

[MartinForReal]

Closing as a duplicate of #2672, which is the more complete implementation of #2458 — it additionally wires OTEL_METRICS_ENABLED / OTLP metrics env through helm/kagent/templates/controller-configmap.yaml so the feature is operable end-to-end, and has passing tests + clean ruff. Keeping a single PR for #2458 to avoid duplicate review. All discussion continues in #2672.