runtime/metrics: fix panic message for Float64Histogram

The panic message erroneously refers to float64 values.

Change-Id: I83380f41d6c28a72bc69a94b9bcdf9d42b1503c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/281236
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

Author: mknyszek

src/runtime/metrics/value.go

@@ -63,7 +63,7 @@ func (v Value) Float64() float64 {
 // If v.Kind() != KindFloat64Histogram, this method panics.
 func (v Value) Float64Histogram() *Float64Histogram {
 	if v.kind != KindFloat64Histogram {
-		panic("called Float64 on non-float64 metric value")
+		panic("called Float64Histogram on non-Float64Histogram metric value")
 	}
 	return (*Float64Histogram)(v.pointer)
 }