PromQL: Fix parser benchmark (#6495)

The parser benchmarks called the `ParseMetric` function instead of the `ParseExpr` function, which resulted in parsing failing every time.

This means only the case of PromQL parser failure was benchmarked.

Signed-off-by: Tobias Guggenmos <tguggenm@redhat.com>

promql/bench_test.go

@@ -250,7 +250,7 @@ func BenchmarkParser(b *testing.B) {
 		b.Run(c, func(b *testing.B) {
 			b.ReportAllocs()
 			for i := 0; i < b.N; i++ {
-				ParseMetric(c)
+				ParseExpr(c)
 			}
 		})
 	}
@@ -259,7 +259,7 @@ func BenchmarkParser(b *testing.B) {
 		b.Run(name, func(b *testing.B) {
 			b.ReportAllocs()
 			for i := 0; i < b.N; i++ {
-				ParseMetric(c)
+				ParseExpr(c)
 			}
 		})
 	}