chore: make tracing optional

[tottoto]

Resolves #713.

[gituser-rs]

is there any chance of approving this?

[seanmonstar]

It will be, it's just a breaking change.

[incisakura]

Waiting for this PR to be merged.

If anyone is annoyed by h2 (or any other crate) emitting tracing logs but actually useless to them, a temporary workaround is to set the tracing feature "max_level_off," which would prevent the generation of the event log code, potentially improving runtime performance and reducing the binary size.

[dependencies]
# your dependencies here

tracing = { version = "*", default-features = false, features = ["max_level_off", "release_max_level_off"] }