Override default brotli compression level 11 -> 4

[Ameobea]

Additional discussion: #287

Motivation

The brotli crate used by async-compression has a default compression level of 11, which is the maximum for brotli. This causes extremely slow compression performance for many response bodies and is definitely an inappropriate compression level for dynamic content.

There's currently an open issue (dropbox/rust-brotli#93) on the brotli crate's repo to change this default, but it hasn't happened at this time.

Solution

This change adds a special case to convert a provided compression level of default to a compression level of 4, which is a reasonable default for dynamic content.

[Ameobea]

An alternative way of implementing this that I just considered is altering the into_async_compression function. We could create a configurable default level for each compression algorithm and then pass the algorithm type as an argument to the function.

It's a bit less hacky and allows us to close the loop for all the other algorithms as well. Will have a bigger impact though.
Lmk if you'd prefer that and I can change over to doing that instead.

[Ameobea]

@davidpdrsn bumping this; I think this is something that should def. still go in.

Lmk if you'd like this implemented a different way or something like that.

[Logarithmus]

@davidpdrsn Recently we discovered a sudden performance drop in our axum web API and after troubleshooting pin-pointed it to clients switching from gzip to brotli, and further investigation led us here. For now we've overriden CompressionLayer::quality to fastest, but it would be nice to merge this so other people won't suddenly encounter this issue like us.

3 months ago the dropbox/rust-brotli#93 has been created, but no reply at all. Also latest commit has been back in January. Looks like fixing this upstream won't happen. So I think you should accept this PR and create a new patch release 0.4.2, so the whole rust web ecosystem could benefit from it.