Add square root function

rules/ast/functions.go

@@ -500,6 +500,17 @@ func log10Impl(timestamp clientmodel.Timestamp, args []Node) interface{} {
 	return vector
 }
 
+// === sqrt(vector VectorNode) Vector ===
+func sqrtImpl(timestamp clientmodel.Timestamp, args []Node) interface{} {
+	n := args[0].(VectorNode)
+	vector := n.Eval(timestamp)
+	for _, el := range vector {
+		el.Metric.Delete(clientmodel.MetricNameLabel)
+		el.Value = clientmodel.SampleValue(math.Sqrt(float64(el.Value)))
+	}
+	return vector
+}
+
 // === deriv(node MatrixNode) Vector ===
 func derivImpl(timestamp clientmodel.Timestamp, args []Node) interface{} {
 	matrixNode := args[0].(MatrixNode)
@@ -721,6 +732,12 @@ var functions = map[string]*Function{
 		returnType: VectorType,
 		callFn:     sortDescImpl,
 	},
+	"sqrt": {
+		name:       "sqrt",
+		argTypes:   []ExprType{VectorType},
+		returnType: VectorType,
+		callFn:     sqrtImpl,
+	},
 	"sum_over_time": {
 		name:       "sum_over_time",
 		argTypes:   []ExprType{MatrixType},

rules/rules_test.go

@@ -1197,6 +1197,13 @@ func TestExpressions(t *testing.T) {
 				`{group="canary", instance="0", job="api-server"} => NaN @[%v]`,
 			},
 		},
+    {
+			expr: `sqrt(vector_matching_a)`,
+			output: []string{
+				`{l="x"} => 3.1622776601683795 @[%v]`,
+				`{l="y"} => 4.47213595499958 @[%v]`,
+			},
+    },
 		{
 			expr: `exp(vector_matching_a)`,
 			output: []string{