Add metrics for APIBinding and APIExport usage data

[Elbehery]

Implements the metrics proposed in #4041. Adds five new Prometheus metrics to improve
observability of APIBinding and APIExport lifecycle and health, following the same
patterns already used for kcp_workspace_count and kcp_logicalcluster_count.

All metrics are system-wide aggregates with no per-resource labels to avoid cardinality
explosion.

New metrics

Metric	Type	Labels	Description
kcp_apibinding_phase	Gauge	phase	Number of APIBindings in each phase (Binding, Bound, or empty for newly created)
kcp_apibinding_condition_status	Gauge	condition, status	Number of APIBindings with each condition type and status (True/False/Unknown)
kcp_apibinding_ready_duration_ms	Histogram	—	Milliseconds from APIBinding creation to reaching the Bound phase
kcp_apiexport_condition_status	Gauge	condition, status	Number of APIExports with each condition type and status
kcp_apiexport_ready_duration_ms	Histogram	—	Milliseconds from APIExport creation to becoming fully operational (IdentityValid=True and VirtualWorkspaceURLsReady=True)

Implementation

• Phase and condition state is tracked per resource key in a sync.Mutex-protected map
  in each controller; on update only the diff is applied to the gauges.
• Histogram observations are idempotent — each resource contributes exactly one sample
  (guarded by a phase-transition check for bindings, a readyAPIExports set for exports)
  so controller restarts do not double-count.
• Each metric is introduced in its own commit with unit tests covering add, update,
  delete, and concurrency cases.

cc @mjudeikis @ntnn @SimonTheLeg

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