feat: TraceQL metrics: average over time

docs/sources/tempo/traceql/metrics-queries.md

@@ -62,6 +62,9 @@ These functions can be added as an operator at the end of any TraceQL query.
 `max_over_time`
 : Returns the maximum value of matching spans values per time interval (see the `step` API parameter)
 
+`avg_over_time`
+: Returns the average value of matching spans values per time interval (see the `step` API parameter)
+
 `quantile_over_time`
 : The quantile of the values in the specified interval
 
@@ -97,7 +100,7 @@ down by HTTP route.
 This might let you determine that `/api/sad` had a higher rate of erroring
 spans than `/api/happy`, for example.
 
-### The `count_over_time`, `min_over_time` and `max_over_time` functions
+### The `count_over_time`, `min_over_time`, `max_over_time` and `avg_over_time` functions
 
 The `count_over_time()` let you counts the number of matching spans per time interval.
 
@@ -128,6 +131,16 @@ The `max_over_time()` let you aggregate numerical values by computing the maximu
 { name = "GET /:endpoint" } | max_over_time(span.http.status_code)
 ```
 
+The `avg_over_time()` let you aggregate numerical values by computing the average value of them, such as the all important span duration.
+
+```
+{ name = "GET /:endpoint" } | avg_over_time(duration) by (span.http.target)
+```
+
+```
+{ name = "GET /:endpoint" } | avg_over_time(event:cpu_seconds_tota)
+```
+
 ### The `quantile_over_time` and `histogram_over_time` functions
 
 The `quantile_over_time()` and `histogram_over_time()` functions let you aggregate numerical values, such as the all important span duration.

pkg/traceql/engine_metrics.go

@@ -17,6 +17,8 @@ import (
 )
 
 const (
+	internalLabelMetaType = "__meta_type"
+	internalMetaTypeCount = "__count"
 	internalLabelBucket   = "__bucket"
 	maxExemplars          = 100
 	maxExemplarsPerBucket = 2
@@ -176,10 +178,12 @@ func (ls Labels) String() string {
 			promValue = "<nil>"
 		case l.Value.Type == TypeString:
 			s := l.Value.EncodeToString(false)
-			if s != "" {
-				promValue = s
-			} else {
+			if s == "nil" {
+				promValue = "<nil>"
+			} else if s == "" {
 				promValue = "<empty>"
+			} else {
+				promValue = s
 			}
 		default:
 			promValue = l.Value.EncodeToString(false)