Avoid making a subscope in tallyMetricsHandler if possible

[k24dizzle]

Creating a subscope, even if it is tmp.Scope.Tagged(nil), takes extra time.

Used this script to benchmark.

Before:

go test -benchmem -run=^$ -bench . -benchtime=10s
goos: darwin
goarch: arm64
pkg: test/prom13
BenchmarkCounter-10     67209068               170.4 ns/op            48 B/op          2 allocs/op
PASS
ok      test/prom13     11.812s

After:

go test -benchmem -run=^$ -bench . -benchtime=10s
goos: darwin
goarch: arm64
pkg: test/prom13
BenchmarkCounter-10     84332931               138.2 ns/op            48 B/op          2 allocs/op
PASS
ok      test/prom13     12.028s