Key construction from labels always allocate

[jean-airoldie]

Currently creating a key from labels will allocate a vector every time, even if the labels are &'static str.

metrics/metrics-core/src/lib.rs

Line 98 in 2f3e344

pub fn from_name_and_labels<N, L>(name: N, labels: L) -> Self

metrics/metrics-core/src/lib.rs

Line 211 in 2f3e344

pub trait IntoLabels {

This means that the metrics macros might potentially allocate at every call. Obviously this is not ideal.

A solution from metrics side would be for a Key to be constructible with a label without any allocation. So in the same sense that name is a ScopedString, the labels could something like:
Cow<&'static, [&'static str]>. So for the special case where the labels are a slice of &'static str this would not allocate. I think this would be ideal.

[tobz]

This will be solved by #80 which caches callsite identifiers when labels are entirely static.

[tobz]

We've reworked how keys are handled in #87, but this is still covered, ultimately, by the caching mechanism introduced in #80, so I'm going to close this.