logging: reduce the amount of allocations in hotpath

[dfreese]

This removes the to_string() call on Priority by adding a private member function numeric_level(). I've implemented the numeric_level() as a private member function to avoid adding to the API surface of Priority. The provided test ensures that the function used internally and the representation publicly, via From, match. Since rust does not have a small string optimization, this avoids an allocation for each call to journal_send.

This also adds a reserve call prior to adding data to the vector to avoid multiple allocations when extend and push are called, since the length of the data being added is fully known.

[lucab]

Thanks for the PR. This could have been probably better split in two commits, but I'm fine reviewing/merging it as a whole too. The change on Priority seems good, but I'm going to take a bit more time to reason about the extend() change as it seems a bit convoluted.

Out of curiosity, did you see this hotspot showing up on some profiling/tracing session or are you just diving through the codebase?

[dfreese]

Out of curiosity, did you see this hotspot showing up on some profiling/tracing session or are you just diving through the codebase?

I don't have benchmarks for this, but I have seen similar make a difference for logging-related that can be called quite often.

[lucab]

With the smaller scope of changes here (Priority &str only), I'm inclined to just accept it and push the extend() changes/discussion to another PR. @swsnr sounds fine?

[swsnr]

I've one question left about the numeric_level function, but other than that I like the current changes. The explicit reserve is a much less invasive change, has a clear benefit, and makes the intention of avoiding allocations explicit. @dfreese Nice change, thank you 🙏🏿