Align compression levels with Envoy

[anuraaga]

Currently, gzip compression, our default, uses Python's default of level 9 which is for long term storage like OS packages and not good for network servers.

I wanted to align with some standard for defaults, and went with Envoy where brotli=3, gzip=6, zstd=3

For reference, connect-go uses the defaults for the Go libraries for all compressions (zstd / brotli are within the conformance runner), which are level=6 for gzip and brotli, and default (level 3) for zstd.

As gzip is used by default, this is probably closer to a bug than performance enhancement

#96

[Zaczero]

It's not a full review but just reading the PR body. brotli-6 is about 2x slower than gzip-6 and CDNs like Cloudflare default to brotli-4 which seems to match the compression time.

https://dev.to/coolblue/improving-website-performance-with-brotli-5h70

[anuraaga]

Thanks @Zaczero for bringing that up - I found https://blog.cloudflare.com/results-experimenting-brotli/ for more background and it's interesting that they default to gzip=8. I interpret this as their gzip default coming from a day of mostly static content and brotli from more dynamic content.

For the defaults, I'd like to pick a reference point and just go with it rather than try to introduce opinions. I remembered Envoy supports all three, and its use case is similar to connect, often serving APIs. So I went with it. But given all the different defaults out there, it looks like going forward it will indeed be important to provided configurable compression to opt-out of our defaults.