fix(k8s-executor): add latency + status metrics around pod API calls

[1fanwang]

Problem

The KubernetesExecutor calls create_namespaced_pod, delete_namespaced_pod, and patch_namespaced_pod against the API server on every task lifecycle event, but emits no metrics around those calls. When a cluster's control plane is slow, throttling (HTTP 429), or returning 5xx, the only signal today is scheduler log noise — there's no way to alert on latency drift or error-rate spikes without scraping logs.

Grafana Dashboard graphing these metrics from our internal fork

Fix

Wrap each of the three pod API call sites in kubernetes_executor_utils.py with Stats.timer for latency (kubernetes_executor.pod_creation / pod_deletion / pod_patching) and a paired Stats.incr tagged by status (pod_creation_status / pod_deletion_status / pod_patching_status). The counter is tagged status="200" on success and with the ApiException.status value on failure, so operators can chart per-status-code rates. The 404-is-fine branch in delete_pod and the swallow-on-failure branches in the two patch methods still behave as before — they just emit a counter on the way out.

The three new timers and three new counters are registered in shared/observability/src/airflow_shared/observability/metrics/metrics_template.yaml so they pass the metrics-registry pre-commit hook and show up in the published metrics docs.

Tests

New unit tests in test_kubernetes_executor.py mock the Stats module and assert the timer + tagged counter fire on both the success path and an ApiException(status=429) failure path for delete_pod.

Closes #66799