feat: add median timing calculation to statsd input plugin

plugins/inputs/statsd/README.md

@@ -183,6 +183,8 @@ metric type:
         for that stat during that interval.
     - `statsd_<name>_mean`: The mean is the average of all values statsd saw
         for that stat during that interval.
+    - `statsd_<name>_median`: The median is the middle of all values statsd saw
+        for that stat during that interval.
     - `statsd_<name>_stddev`: The stddev is the sample standard deviation
         of all values statsd saw for that stat during that interval.
     - `statsd_<name>_sum`: The sum is the sample sum of all values statsd saw

plugins/inputs/statsd/running_stats.go

@@ -7,6 +7,7 @@ import (
 )
 
 const defaultPercentileLimit = 1000
+const defaultMedianLimit = 1000
 
 // RunningStats calculates a running mean, variance, standard deviation,
 // lower bound, upper bound, count, and can calculate estimated percentiles.
@@ -29,14 +30,22 @@ type RunningStats struct {
 	lower float64
 	upper float64
 
+	// Array used to calculate estimated median values
+	// We will store a maximum of MedLimit values, at which point we will start
+	// slicing old values
+	med      []float64
+	MedLimit int
+
 	// cache if we have sorted the list so that we never re-sort a sorted list,
 	// which can have very bad performance.
-	sorted bool
+	SortedPerc bool
+	SortedMed  bool
 }
 
 func (rs *RunningStats) AddValue(v float64) {
 	// Whenever a value is added, the list is no longer sorted.
-	rs.sorted = false
+	rs.SortedPerc = false
+	rs.SortedMed = false
 
 	if rs.n == 0 {
 		rs.k = v
@@ -45,7 +54,11 @@ func (rs *RunningStats) AddValue(v float64) {
 		if rs.PercLimit == 0 {
 			rs.PercLimit = defaultPercentileLimit
 		}
+		if rs.MedLimit == 0 {
+			rs.MedLimit = defaultMedianLimit
+		}
 		rs.perc = make([]float64, 0, rs.PercLimit)
+		rs.med = make([]float64, 0, rs.MedLimit)
 	}
 
 	// These are used for the running mean and variance
@@ -69,12 +82,36 @@ func (rs *RunningStats) AddValue(v float64) {
 		// Reached limit, choose random index to overwrite in the percentile array
 		rs.perc[rand.Intn(len(rs.perc))] = v
 	}
+
+	if len(rs.med) < rs.MedLimit {
+		rs.med = append(rs.med, v)
+	} else {
+		// Reached limit, slice off first element
+		rs.med = rs.med[1:]
+		rs.med[len(rs.med)] = v
+	}
 }
 
 func (rs *RunningStats) Mean() float64 {
 	return rs.k + rs.ex/float64(rs.n)
 }
 
+func (rs *RunningStats) Median() float64 {
+	count := len(rs.med)
+	// Need to sort for median
+	if !rs.SortedMed {
+		sort.Float64s(rs.med)
+		rs.SortedMed = true
+	}
+	if count == 0 {
+		return 0
+	} else if count%2 == 0 {
+		return (rs.med[count/2-1] + rs.med[count/2]) / 2
+	} else {
+		return rs.med[count/2]
+	}
+}
+
 func (rs *RunningStats) Variance() float64 {
 	return (rs.ex2 - (rs.ex*rs.ex)/float64(rs.n)) / float64(rs.n)
 }
@@ -104,9 +141,9 @@ func (rs *RunningStats) Percentile(n float64) float64 {
 		n = 100
 	}
 
-	if !rs.sorted {
+	if !rs.SortedPerc {
 		sort.Float64s(rs.perc)
-		rs.sorted = true
+		rs.SortedPerc = true
 	}
 
 	i := float64(len(rs.perc)) * n / float64(100)

plugins/inputs/statsd/statsd.go

@@ -253,6 +253,7 @@ func (s *Statsd) Gather(acc telegraf.Accumulator) error {
 				prefix = fieldName + "_"
 			}
 			fields[prefix+"mean"] = stats.Mean()
+			fields[prefix+"median"] = stats.Median()
 			fields[prefix+"stddev"] = stats.Stddev()
 			fields[prefix+"sum"] = stats.Sum()
 			fields[prefix+"upper"] = stats.Upper()

plugins/inputs/statsd/statsd_test.go

@@ -419,6 +419,7 @@ func TestParse_Timings(t *testing.T) {
 		"count":         int64(5),
 		"lower":         float64(1),
 		"mean":          float64(3),
+		"median":        float64(1),
 		"stddev":        float64(4),
 		"sum":           float64(15),
 		"upper":         float64(11),
@@ -913,6 +914,7 @@ func TestParse_DataDogTags(t *testing.T) {
 						"count":  10,
 						"lower":  float64(3),
 						"mean":   float64(3),
+						"median": float64(3),
 						"stddev": float64(0),
 						"sum":    float64(30),
 						"upper":  float64(3),
@@ -1211,6 +1213,7 @@ func TestParse_TimingsMultipleFieldsWithTemplate(t *testing.T) {
 		"success_count":         int64(5),
 		"success_lower":         float64(1),
 		"success_mean":          float64(3),
+		"success_median":        float64(1),
 		"success_stddev":        float64(4),
 		"success_sum":           float64(15),
 		"success_upper":         float64(11),
@@ -1219,6 +1222,7 @@ func TestParse_TimingsMultipleFieldsWithTemplate(t *testing.T) {
 		"error_count":         int64(5),
 		"error_lower":         float64(2),
 		"error_mean":          float64(6),
+		"error_median":        float64(2),
 		"error_stddev":        float64(8),
 		"error_sum":           float64(30),
 		"error_upper":         float64(22),
@@ -1259,6 +1263,7 @@ func TestParse_TimingsMultipleFieldsWithoutTemplate(t *testing.T) {
 		"count":         int64(5),
 		"lower":         float64(1),
 		"mean":          float64(3),
+		"median":        float64(1),
 		"stddev":        float64(4),
 		"sum":           float64(15),
 		"upper":         float64(11),
@@ -1268,6 +1273,7 @@ func TestParse_TimingsMultipleFieldsWithoutTemplate(t *testing.T) {
 		"count":         int64(5),
 		"lower":         float64(2),
 		"mean":          float64(6),
+		"median":        float64(2),
 		"stddev":        float64(8),
 		"sum":           float64(30),
 		"upper":         float64(22),