Add build flag to disable labels

[fralalonde]

Metric Labels have a small runtime performance impact even if they're not used:
With labels

test aggregate::bucket::bench::aggregate_counter   ... bench:          17 ns/iter (+/- 0)
test aggregate::bucket::bench::aggregate_marker    ... bench:          10 ns/iter (+/- 1)
test core::bench::time_bench_direct_dispatch_event ... bench:          10 ns/iter (+/- 0)
test core::proxy::bench::proxy_marker_to_aggregate ... bench:          29 ns/iter (+/- 0)
test core::proxy::bench::proxy_marker_to_void      ... bench:          28 ns/iter (+/- 1)

Without labels:

test aggregate::bucket::bench::aggregate_counter   ... bench:          13 ns/iter (+/- 0)
test aggregate::bucket::bench::aggregate_marker    ... bench:           8 ns/iter (+/- 1)
test core::bench::time_bench_direct_dispatch_event ... bench:           8 ns/iter (+/- 0)
test core::proxy::bench::proxy_marker_to_aggregate ... bench:          22 ns/iter (+/- 0)
test core::proxy::bench::proxy_marker_to_void      ... bench:          22 ns/iter (+/- 4)

This is possibly due to the extra parameter being passed around. A build flag could disable the use of labels to regain the extra perf when they're not needed.

[fralalonde]

Turns out labels have an even bigger perf cost but which is mostly optimized away if they aren't used as in the benchmarks above. Not sure there's a way around it other than creating a completely separate call path, which is more trouble than its worth. If you need labels, you pay for labels.

test aggregate::bucket::bench::aggregate_counter   ... bench:          82 ns/iter (+/- 4)
test aggregate::bucket::bench::aggregate_marker    ... bench:          77 ns/iter (+/- 21)
test core::bench::time_bench_direct_dispatch_event ... bench:          77 ns/iter (+/- 4)
test core::proxy::bench::proxy_marker_to_aggregate ... bench:          94 ns/iter (+/- 3)
test core::proxy::bench::proxy_marker_to_void      ... bench:          98 ns/iter (+/- 4)