metricsql: add function histogram_fraction()

Add support for the histogram_fraction() function in MetricsQL. It is registered as a transform and recognized by the optimizer (arg index 2) so queries using it parse and optimize correctly.

PR #59
Related to VictoriaMetrics/VictoriaMetrics#5346

Author: Haleygo

optimizer.go

@@ -700,7 +700,7 @@ func getTransformArgIdxForOptimization(funcName string, args []Expr) int {
 		return -1
 	case "end", "now", "pi", "ru", "start", "step", "time":
 		return -1
-	case "limit_offset":
+	case "limit_offset", "histogram_fraction":
 		return 2
 	case "buckets_limit", "histogram_quantile", "histogram_share", "range_quantile",
 		"range_trim_outliers", "range_trim_spikes", "range_trim_zscore":

transform.go

@@ -35,6 +35,7 @@ var transformFuncs = map[string]bool{
 	"exp":                        true,
 	"floor":                      true,
 	"histogram_avg":              true,
+	"histogram_fraction":         true,
 	"histogram_quantile":         true,
 	"histogram_quantiles":        true,
 	"histogram_share":            true,