change the order of summary and quantile(s) (#105)

This is a follow-up to
#99.

Prior to #99, the order of summary metrics (`_sum` and `_count`) and
quantiles could vary depending on the metric name.

After #99, the order was fixed to show summaries first, followed by
quantiles. This ordering was based on the author's tests, but at the
time, the author was unaware that different orders were possible.

In conclusion:
1. The order should be fixed, which was accomplished in the previous
pull request.
2. The fixed order should match Prometheus or the most common ordering
from versions before #99.
    - The current version (v1.40.1) fixed this order in an unusual way.

Therefore, this pull request modifies the sorting logic for summaries
and quantiles, enforcing that quantiles are printed first, followed by
summaries.

---------

Co-authored-by: Max Kotliar <[email protected]>

Author: jiekun

set.go

@@ -47,10 +47,10 @@ func (s *Set) WritePrometheus(w io.Writer) {
 			return fName1 < fName2
 		}
 
+		// Only summary and quantile(s) have different metric types.
+		// Sorting by metric type will stabilize the order for summary and quantile(s).
 		mType1 := s.a[i].metric.metricType()
 		mType2 := s.a[j].metric.metricType()
-
-		// stabilize the order for summary and quantiles.
 		if mType1 != mType2 {
 			return mType1 < mType2
 		}

set_example_test.go

@@ -69,22 +69,22 @@ func ExampleExposeMetadata() {
 	// request_duration_seconds_count{path="/foo/bar"} 2
 	// # HELP response_size_bytes
 	// # TYPE response_size_bytes summary
-	// response_size_bytes_sum 1
-	// response_size_bytes_count 1
 	// response_size_bytes{quantile="0.5"} 1
 	// response_size_bytes{quantile="0.9"} 1
 	// response_size_bytes{quantile="0.97"} 1
 	// response_size_bytes{quantile="0.99"} 1
 	// response_size_bytes{quantile="1"} 1
+	// response_size_bytes_sum 1
+	// response_size_bytes_count 1
 	// # HELP response_size_bytes_extra_suffix
 	// # TYPE response_size_bytes_extra_suffix summary
-	// response_size_bytes_extra_suffix_sum 1
-	// response_size_bytes_extra_suffix_count 1
 	// response_size_bytes_extra_suffix{quantile="0.5"} 1
 	// response_size_bytes_extra_suffix{quantile="0.9"} 1
 	// response_size_bytes_extra_suffix{quantile="0.97"} 1
 	// response_size_bytes_extra_suffix{quantile="0.99"} 1
 	// response_size_bytes_extra_suffix{quantile="1"} 1
+	// response_size_bytes_extra_suffix_sum 1
+	// response_size_bytes_extra_suffix_count 1
 	// # HELP set_counter
 	// # TYPE set_counter counter
 	// set_counter 1
@@ -97,11 +97,11 @@ func ExampleExposeMetadata() {
 	// used_bytes{foo="baz"} 43
 	// # HELP vm_request_duration_seconds
 	// # TYPE vm_request_duration_seconds summary
-	// vm_request_duration_seconds_sum{path="/api/v1/query_range"} 1
-	// vm_request_duration_seconds_count{path="/api/v1/query_range"} 1
 	// vm_request_duration_seconds{path="/api/v1/query_range",quantile="0.5"} 1
 	// vm_request_duration_seconds{path="/api/v1/query_range",quantile="0.9"} 1
 	// vm_request_duration_seconds{path="/api/v1/query_range",quantile="0.97"} 1
 	// vm_request_duration_seconds{path="/api/v1/query_range",quantile="0.99"} 1
 	// vm_request_duration_seconds{path="/api/v1/query_range",quantile="1"} 1
+	// vm_request_duration_seconds_sum{path="/api/v1/query_range"} 1
+	// vm_request_duration_seconds_count{path="/api/v1/query_range"} 1
 }

summary.go

@@ -120,7 +120,13 @@ func (sm *Summary) marshalTo(prefix string, w io.Writer) {
 }
 
 func (sm *Summary) metricType() string {
-	return "summary"
+	// this metric type should not be printed, because summary (sum and count)
+	// of the same metric family will be printed after quantile(s).
+	// If metadata is needed, the metadata from quantile(s) should be used.
+	// quantile will be printed first, so its metrics type won't be printed as metadata.
+	// Printing quantiles before sum and count aligns this code with Prometheus behavior.
+	// See: https://github.com/VictoriaMetrics/metrics/pull/99
+	return "unsupported"
 }
 
 func splitMetricName(name string) (string, string) {
@@ -201,11 +207,7 @@ func (qv *quantileValue) marshalTo(prefix string, w io.Writer) {
 }
 
 func (qv *quantileValue) metricType() string {
-	// this metricsType should not be printed, because summary (sum and count) of the same metric family will be printed first,
-	// and if metadata is needed, the metadata from summary should be used.
-	// quantile will be printed later, so its metrics type won't be printed as metadata.
-	// See: https://github.com/VictoriaMetrics/metrics/pull/99
-	return "unsupported"
+	return "summary"
 }
 
 func addTag(name, tag string) string {