Performance optimizations

[kibertoad]

Having encountered performance degradation after introducing fastify-metrics, we've attempted doing some of the obvious performance optimizations.

Here are the results (benchmarks included):
kibertoad/fastify-prometheus-nitro#1

I'm not sending PR as-is because it is pretty intrusive, but I can replicate same or all of it in original repo if perf increase seems significant enough for you, and changes acceptable.

[SkeLLLa]

Hi, since this plugin provide some utility functions the less if affects app performance, better it becomes. So any performance improvements are more than welcome.

[kibertoad]

Awesome, I'll try to make a set of incremental PRs then.

[kibertoad]

@SkeLLLa PR is up!