docs: add 4 undocumented metrics to docs/metrics.md

[ChinonsoNwakudu]

Changes

Add 4 metrics that are defined in the controller code but were missing from docs/metrics.md:

• tekton_pipelines_controller_running_pipelineruns_waiting_on_pipeline_resolution
• tekton_pipelines_controller_running_pipelineruns_waiting_on_task_resolution
• tekton_pipelines_controller_running_taskruns_waiting_on_task_resolution_count
• tekton_pipelines_controller_taskruns_pod_latency_milliseconds (includes note on unbounded cardinality via the pod label — see taskruns_pod_latency_milliseconds metric has unbounded cardinality due to hardcoded pod label #9393)

All 4 are Gauge metrics at experimental status. New rows are inserted after throttled_by_node and before client_latency.

Fixes #9495

Submitter Checklist

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
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Release Notes

NONE

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