A couple tweaks to HookMetricEmit documentation

hook_defaults_funcs.go

@@ -29,6 +29,11 @@ func (f HookInsertBeginFunc) IsPlugin() bool { return true }
 
 // HookMetricEmitFunc is a convenience helper for implementing
 // rivertype.HookMetricEmit using a simple function instead of a struct.
+//
+// Notably, this function is invoked each time River emits a metric. Metrics are
+// emitted in very hot paths like job fetching, and should therefore not block
+// on network I/O or anything else, and should usually pass metrics through to
+// an asynchronous instrumentation package like OpenTelemetry.
 type HookMetricEmitFunc func(ctx context.Context, params *rivertype.HookMetricEmitParams)
 
 func (f HookMetricEmitFunc) IsHook() bool { return true }

rivertype/river_type.go

@@ -354,8 +354,8 @@ type HookInsertBegin interface {
 type HookMetricEmit interface {
 	Hook
 
-	// MetricEmit is invoked each time River emits a metric. This is invoked in
-	// very hot paths like job fetching, and should therefore not block on
+	// MetricEmit is invoked each time River emits a metric. Metrics are emitted
+	// in very hot paths like job fetching, and should therefore not block on
 	// network I/O or anything else, and should usually pass metrics through to
 	// an asynchronous instrumentation package like OpenTelemetry.
 	MetricEmit(ctx context.Context, params *HookMetricEmitParams)