Feat/metrics doc generator v3

[ivankatliarchuk]

Why: The v1 generator required manually seeding GaugeVec label values before Gather() to make them visible, and relied on a static MetricRegistry for shared/conditional
metrics. This approach breaks silently when label names change — the docs drift without any signal.

What:

Change all prober function signatures from *prometheus.Registry → prometheus.Registerer (non-breaking, callers unchanged)
Add internal/gen/docs/metrics_v3/ with a capturingRegistry that intercepts every MustRegister call and invokes Describe() on the collector — extracting label names
without needing observed values
GaugeVec labels (e.g. probe_grpc_healthcheck_response.serving_status, probe_websocket_duration_seconds.phase) now come directly from code, not manual declarations
Conditional metrics that only register on live code paths (TLS, CEL, SOA, ICMP reply) still fall back to prober.MetricRegistry
Golden-file test in main_test.go fails when any metric name, type, labels, or help text changes — making docs drift visible in CI

Branch: feat/metrics-doc-generator-v3

Title: feat: add capturingRegistry metrics doc generator with golden-file test

Why: The v1 generator required manually seeding GaugeVec label values before Gather() to make them visible, and relied on a static MetricRegistry for shared/conditional
metrics. This approach breaks silently when label names change — the docs drift without any signal.

What:

• Change all prober function signatures from *prometheus.Registry → prometheus.Registerer (non-breaking, callers unchanged)
• Add internal/gen/docs/metrics_v3/ with a capturingRegistry that intercepts every MustRegister call and invokes Describe() on the collector — extracting label names
  without needing observed values
• GaugeVec labels (e.g. probe_grpc_healthcheck_response.serving_status, probe_websocket_duration_seconds.phase) now come directly from code, not manual declarations
• Conditional metrics that only register on live code paths (TLS, CEL, SOA, ICMP reply) still fall back to prober.MetricRegistry
• Golden-file test in main_test.go fails when any metric name, type, labels, or help text changes — making docs drift visible in CI