optimize header name handling in Grpc::map_response

[srijs]

Avoid parsing reserved header names on every call to MetadataMap::into_sanitized_headers as well as in Status::to_http. More explanation below. The change should be relatively straight forward and low-complexity, but let me know if you'd like to see an isolated benchmark for this regardless!

Motivation

The MetadataMap::into_sanitized_headers function is called for every response transitively through Grpc::map_response and Response::into_http.

The way that it currently sanitizes headers by removing the reserved GRPC headers involves calling HeaderMap::remove with a &str key, which results in the string key being parsed into a header name at runtime for every one of the reserved headers (see the call here).

This makes it show up on my profiles in the single-digit percentage range, which seemed worthwhile improving to me, especially considering the low complexity of the change.

A similar situation exists in Status::to_http where the GRPC status header names are being parsed at runtime when inserted into the header map.

Solution

The solution implemented is to parse the reserved headers at compile time using the const function HeaderName::from_static. This eliminates any runtime overhead of parsing the reserved header names.

[srijs]

@LucioFranco this should be a relatively simple chance, any indication as to whether you'd be able to merge it or have any feedback would be appreciated, thanks!

[tottoto]

Thank you!