Reduce regression

Signed-off-by: Marco Pracucci <[email protected]>

tsdb/postings_for_matchers_cache.go

@@ -187,6 +187,12 @@ func (p *postingsForMatcherPromise) result(ctx context.Context) (index.Postings,
 		if p.err != nil {
 			return nil, fmt.Errorf("postingsForMatchers promise completed with error: %w", p.err)
 		}
+
+		trace.SpanFromContext(ctx).AddEvent("completed postingsForMatchers promise", trace.WithAttributes(
+			// Do not format the timestamp to introduce a performance regression.
+			attribute.Int64("evaluation completed at (epoch seconds)", p.evaluationCompletedAt.Unix()),
+		))
+
 		return p.cloner.Clone(), nil
 	}
 }
@@ -220,7 +226,6 @@ func (c *PostingsForMatchersCache) postingsForMatchersPromise(ctx context.Contex
 		promise.callersCtxTracker.close()
 
 		oldPromise := oldPromiseValue.(*postingsForMatcherPromise)
-		oldPromiseEvaluationCompletedAt := time.Time{}
 
 		// Check if the promise already completed the execution and its TTL has not expired yet.
 		// If the TTL has expired, we don't want to return it, so we just recompute the postings
@@ -229,10 +234,6 @@ func (c *PostingsForMatchersCache) postingsForMatchersPromise(ctx context.Contex
 		if c.ttl > 0 {
 			select {
 			case <-oldPromise.done:
-				// Keep track of when the evaluation was completed. This field can be safely accessed only when
-				// the promise has done.
-				oldPromiseEvaluationCompletedAt = oldPromise.evaluationCompletedAt
-
 				if c.timeNow().Sub(oldPromise.evaluationCompletedAt) >= c.ttl {
 					// The cached promise already expired, but it has not been evicted.
 					span.AddEvent("skipping cached postingsForMatchers promise because its TTL already expired", trace.WithAttributes(
@@ -269,8 +270,7 @@ func (c *PostingsForMatchersCache) postingsForMatchersPromise(ctx context.Contex
 		}
 
 		c.metrics.hits.Inc()
-		span.AddEvent("using cached postingsForMatchers promise", trace.WithAttributes(
-			attribute.Stringer("cached promise evaluation completed at", evaluationCompletedAtTime(oldPromiseEvaluationCompletedAt)),
+		span.AddEvent("waiting cached postingsForMatchers promise", trace.WithAttributes(
 			attribute.String("cache_key", key),
 		))
 
@@ -308,16 +308,6 @@ func (c *PostingsForMatchersCache) postingsForMatchersPromise(ctx context.Contex
 	return promise.result
 }
 
-type evaluationCompletedAtTime time.Time
-
-func (t evaluationCompletedAtTime) String() string {
-	if time.Time(t).IsZero() {
-		return "not completed (in-flight)"
-	}
-
-	return time.Time(t).String()
-}
-
 type postingsForMatchersCachedCall struct {
 	key string
 	ts  time.Time