docs: document 4 undocumented metrics in docs/metrics.md

Summary

4 metrics are defined in the controller code but not documented in docs/metrics.md:

| Metric | Defined in | Description | |--------|-----------|-------------| | tekton_pipelines_controller_running_pipelineruns_waiting_on_pipeline_resolution | pkg/pipelinerunmetrics/metrics.go:158 | Gauge for PipelineRuns waiting on pipeline resolution | | tekton_pipelines_controller_running_pipelineruns_waiting_on_task_resolution | pkg/pipelinerunmetrics/metrics.go:167 | Gauge for PipelineRuns waiting on task resolution | | tekton_pipelines_controller_running_taskruns_waiting_on_task_resolution_count | pkg/taskrunmetrics/metrics.go:197 | Gauge for TaskRuns waiting on task resolution | | tekton_pipelines_controller_taskruns_pod_latency_milliseconds | pkg/taskrunmetrics/metrics.go:224 | Gauge for pod scheduling latency (see also #9393 for cardinality concerns) |

What to do

1. Add rows for each missing metric to the table in docs/metrics.md
2. Include the metric type (Gauge), any labels/tags, and status (experimental)
3. For taskruns_pod_latency_milliseconds, note the cardinality concern with the pod label (see #9393)

How to find metric definitions

rg '"tekton_pipelines_controller_' pkg/taskrunmetrics/metrics.go pkg/pipelinerunmetrics/metrics.go

How to verify

• Compare the documented metrics table against the code definitions
• Ensure all metrics from code appear in docs

/kind documentation