Too much time waste on Labels::default

[taikulawo]

We found metrics-tracing-context use too much time on CPU cycle.
The callstack show Labels::default are in busyloop.

metrics/metrics-tracing-context/src/tracing_integration.rs

Line 52 in e6cf124

Labels(get_pool().pull_owned())

It maybe related EVaillant/lockfree-object-pool#5 (comment)

[tobz]

Yeah, lockfree-object-pool is likely not the best possible object pool we could be using here. I'm open to PRs for changing/improving things if you have a workload you can use to demonstrate the improvements to switching to something else.