Don't emit higher cardinality attributes for metrics (#14)

This ones follows up #11. I said in the pull request that higher
cardinality attributes aren't emitted with metrics, but I got that wrong
as I'd forgotten I was feeding all span attributes into metrics as well.

Here, bifurcate attributes so that high cardinality attributes are added
to spans, but not to metrics.

Author: brandur

otelriver/middleware.go

@@ -126,7 +126,8 @@ func (m *Middleware) InsertMany(ctx context.Context, manyParams []*rivertype.Job
 	defer func() {
 		duration := m.durationInPreferredUnit(time.Since(begin))
 
-		setAttributeAndSpanStatus(attrs, statusIndex, span, panicked, err)
+		setStatus(attrs, statusIndex, span, panicked, err)
+		span.SetAttributes(attrs...) // set after finalizing status
 
 		// This allocates a new slice, so make sure to do it as few times as possible.
 		measurementOpt := metric.WithAttributes(attrs...)
@@ -148,17 +149,14 @@ func (m *Middleware) Work(ctx context.Context, job *rivertype.JobRow, doInner fu
 	defer span.End()
 
 	attrs := []attribute.KeyValue{
-		attribute.Int64("id", job.ID),
 		attribute.Int("attempt", job.Attempt),
-		attribute.String("created_at", job.CreatedAt.Format(time.RFC3339)),
 		attribute.String("kind", job.Kind),
 		attribute.Int("priority", job.Priority),
 		attribute.String("queue", job.Queue),
-		attribute.String("scheduled_at", job.ScheduledAt.Format(time.RFC3339)),
 		attribute.String("status", ""), // replaced below
 		attribute.StringSlice("tag", job.Tags),
 	}
-	const statusIndex = 7
+	const statusIndex = 4
 
 	var (
 		begin    = time.Now()
@@ -181,7 +179,19 @@ func (m *Middleware) Work(ctx context.Context, job *rivertype.JobRow, doInner fu
 			}
 		}
 
-		setAttributeAndSpanStatus(attrs, statusIndex, span, panicked, err)
+		setStatus(attrs, statusIndex, span, panicked, err)
+
+		{
+			// Add some higher cardinality attributes to spans, but keep them
+			// out of metrics given it's been traditional wisdom that metric
+			// attribute sets shouldn't be too large.
+			attrs := append(attrs,
+				attribute.Int64("id", job.ID),
+				attribute.String("created_at", job.CreatedAt.Format(time.RFC3339)),
+				attribute.String("scheduled_at", job.ScheduledAt.Format(time.RFC3339)),
+			)
+			span.SetAttributes(attrs...) // set after finalizing status
+		}
 
 		// This allocates a new slice, so make sure to do it as few times as possible.
 		measurementOpt := metric.WithAttributes(attrs...)
@@ -234,7 +244,7 @@ func mustInt64Counter(meter metric.Meter, name string, options ...metric.Int64Co
 // Sets success status on the given span and within the set of attributes. The
 // index of the status attribute is required ahead of time as a minor
 // optimization.
-func setAttributeAndSpanStatus(attrs []attribute.KeyValue, statusIndex int, span trace.Span, panicked bool, err error) {
+func setStatus(attrs []attribute.KeyValue, statusIndex int, span trace.Span, panicked bool, err error) {
 	if attrs[statusIndex].Key != "status" {
 		panic("status attribute not at expected index; bug?") // protect against future regression
 	}
@@ -250,5 +260,4 @@ func setAttributeAndSpanStatus(attrs []attribute.KeyValue, statusIndex int, span
 		attrs[statusIndex] = attribute.String("status", "ok")
 		span.SetStatus(codes.Ok, "")
 	}
-	span.SetAttributes(attrs...) // set after finalizing status
 }