[eno] Adding more Eno metrics

[ruinan-liu]

Add observability metrics for Eno synthesis and reconciliation

Implements the gaps documented in docs/observability-gaps.md.

New metrics:

• eno-controller:
  • eno_synthesis_result_total{synthesizer,result}
  • eno_synthesis_duration_seconds{synthesizer,result}
  • eno_composition_synthesis_wait_seconds
• eno-reconciler:
  • eno_reconciliation_workqueue_depth
  • eno_write_buffer_depth
  • eno_write_buffer_status_update_errors_total{op}

Bug fix:

• eno_reconciliation_duration_seconds was observing milliseconds into
  second-named buckets, sending 100% of samples to +Inf. Fixed in place
  since existing data was already unusable.

Implementation notes:

• The two new depth gauges use prometheus.NewGaugeFunc backed by an
  atomic.Pointer[func() int] mailbox so they can be registered at init()
  time before their queue exists.
• Reconciler depth: installed via controller-runtime's NewQueue hook
  on the Controller's TypedOptions; intercepts the same typed queue
  controller-runtime would have constructed by default.
• WriteBuffer depth: installed when the buffer constructs its own
  workqueue in NewResourceSliceWriteBuffer.
• The flowcontrol write buffer's queue was switched from the deprecated
  workqueue.RateLimitingInterface to the typed
  workqueue.TypedRateLimitingInterface[types.NamespacedName], dropping
  the interface{} cast in processQueueItem.