Consider cheaper representations of metric label data

[lukesteensen]

We currently represent metric labels as full Strings. This is a simple and ergonomic choice but has a cost in memory use and copying.

Alternative representations like Atom and Cow-like structures could reduce this cost. With proper benchmarking, we can evaluate performance differences and weight them with concerns like ergonomics to decide if switching is worthwhile.

Originally brought up by @LucioFranco here.

[jszwedko]

Closing since we'll let profiling motivate future performance changes.