updated logic and tests to use arithmetic mean just for NHCB

Signed-off-by: amanycodes <amanycodes@gmail.com>

Author: amanycodes

promql/functions.go

@@ -1367,8 +1367,10 @@ func funcHistogramStdDev(vals []parser.Value, _ parser.Expressions, enh *EvalNod
 			var val float64
 			if bucket.Lower <= 0 && 0 <= bucket.Upper {
 				val = 0
-			} else {
+			} else if sample.H.Schema == -53 {
 				val = (bucket.Upper * bucket.Lower) / 2.0
+			} else {
+				val = math.Sqrt(bucket.Upper * bucket.Lower)
 				if bucket.Upper < 0 {
 					val = -val
 				}

promql/promqltest/testdata/histograms.test

@@ -96,7 +96,7 @@ eval instant at 50m histogram_avg(testhistogram3)
 # Test histogram_stddev. This has no classic equivalent.
 eval instant at 50m histogram_stddev(testhistogram3)
 	{start="positive"} 2.962188994031886
-	{start="negative"} 4.017507000616178
+	{start="negative"} 3.9825070621406313
 
 # Test histogram_stdvar. This has no classic equivalent.
 eval instant at 50m histogram_stdvar(testhistogram3)

promql/promqltest/testdata/native_histograms.test

@@ -313,7 +313,7 @@ load 10m
    histogram_stddev_stdvar_1 {{schema:2 count:4 sum:10 buckets:[1 0 0 0 1 0 0 1 1]}}x1
 
 eval instant at 10m histogram_stddev(histogram_stddev_stdvar_1)
-    {} 2.6436835435182546
+    {} 1.0787993180043811
 
 eval instant at 10m histogram_stdvar(histogram_stddev_stdvar_1)
     {} 6.989062678269235
@@ -325,7 +325,7 @@ load 10m
    histogram_stddev_stdvar_2 {{schema:8 count:10 sum:10 buckets:[1 2 3 4]}}x1
 
 eval instant at 10m histogram_stddev(histogram_stddev_stdvar_2)
-    {} 0.49592215632921693
+    {} 0.0048960313898237465
 
 eval instant at 10m histogram_stdvar(histogram_stddev_stdvar_2)
     {} 0.2459387851382203
@@ -337,7 +337,7 @@ load 10m
    histogram_stddev_stdvar_3 {{schema:3 count:7 sum:62 z_bucket:1 buckets:[0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 ] n_buckets:[0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 ]}}x1
 
 eval instant at 10m histogram_stddev(histogram_stddev_stdvar_3)
-    {} 2067.1553085327896
+    {} 42.94723640025793
 
 eval instant at 10m histogram_stdvar(histogram_stddev_stdvar_3)
     {} 4.273131069595293e+06
@@ -349,7 +349,7 @@ load 10m
    histogram_stddev_stdvar_4 {{schema:0 count:10 sum:-112946 z_bucket:0 n_buckets:[0 0 1 1 1 0 1 1 0 0 3 0 0 0 1 0 0 1]}}x1
 
 eval instant at 10m histogram_stddev(histogram_stddev_stdvar_4)
-    {} 1.3583500773768022e+09
+    {} 27556.344499842402
 
 eval instant at 10m histogram_stdvar(histogram_stddev_stdvar_4)
     {} 1.8451149327095644e+18
@@ -361,7 +361,7 @@ load 10m
    histogram_stddev_stdvar_5 {{schema:0 count:10 sum:-100 z_bucket:0 n_buckets:[0 0 0 0 10]}}x1
 
 eval instant at 10m histogram_stddev(histogram_stddev_stdvar_5)
-    {} 54
+    {} 1.3137084989847612
 
 eval instant at 10m histogram_stdvar(histogram_stddev_stdvar_5)
     {} 2916