feat: add prebuilds reconciliation duration metric (#20535)

## Description

Adds `coderd_prebuilds_reconciliation_duration_seconds` histogram metric
to track the duration of each prebuilds reconciliation cycle.

This metric helps operators monitor reconciliation performance and
identify potential bottlenecks.

## Changes

- Added `ReconcileStats` struct to capture reconciliation cycle
statistics
- Updated `ReconcileAll()` to return stats including elapsed time
- Added histogram metric
`coderd_prebuilds_reconciliation_duration_seconds`

coderd/prebuilds/api.go

@@ -37,13 +37,18 @@ type ReconciliationOrchestrator interface {
 	TrackResourceReplacement(ctx context.Context, workspaceID, buildID uuid.UUID, replacements []*sdkproto.ResourceReplacement)
 }
 
+// ReconcileStats contains statistics about a reconciliation cycle.
+type ReconcileStats struct {
+	Elapsed time.Duration
+}
+
 type Reconciler interface {
 	StateSnapshotter
 
 	// ReconcileAll orchestrates the reconciliation of all prebuilds across all templates.
 	// It takes a global snapshot of the system state and then reconciles each preset
 	// in parallel, creating or deleting prebuilds as needed to reach their desired states.
-	ReconcileAll(ctx context.Context) error
+	ReconcileAll(ctx context.Context) (ReconcileStats, error)
 }
 
 // StateSnapshotter defines the operations necessary to capture workspace prebuilds state.

coderd/prebuilds/noop.go

@@ -17,7 +17,11 @@ func (NoopReconciler) Run(context.Context)         {}
 func (NoopReconciler) Stop(context.Context, error) {}
 func (NoopReconciler) TrackResourceReplacement(context.Context, uuid.UUID, uuid.UUID, []*sdkproto.ResourceReplacement) {
 }
-func (NoopReconciler) ReconcileAll(context.Context) error { return nil }
+
+func (NoopReconciler) ReconcileAll(context.Context) (ReconcileStats, error) {
+	return ReconcileStats{}, nil
+}
+
 func (NoopReconciler) SnapshotState(context.Context, database.Store) (*GlobalSnapshot, error) {
 	return &GlobalSnapshot{}, nil
 }