Drop metric name for "atan2" binary operator

The operator changes the meaning of the metric, so the metric name should
be dropped. Technically this would be a breaking change, but it's also very
obviously a bug and not likely that anyone depends on it.

Signed-off-by: Julius Volz <julius.volz@gmail.com>

promql/engine.go

@@ -2874,7 +2874,7 @@ func btos(b bool) float64 {
 // result of the op operation.
 func shouldDropMetricName(op parser.ItemType) bool {
 	switch op {
-	case parser.ADD, parser.SUB, parser.DIV, parser.MUL, parser.POW, parser.MOD:
+	case parser.ADD, parser.SUB, parser.DIV, parser.MUL, parser.POW, parser.MOD, parser.ATAN2:
 		return true
 	default:
 		return false

promql/testdata/operators.test

@@ -477,10 +477,10 @@ load 5m
     trigNaN{} NaN
 
 eval instant at 5m trigy atan2 trigx
-    trigy{} 0.4636476090008061
+    {} 0.4636476090008061
 
 eval instant at 5m trigy atan2 trigNaN
-    trigy{} NaN
+    {} NaN
 
 eval instant at 5m 10 atan2 20
     0.4636476090008061