[trace] Reduce code bloat of on_xxx function #342

[erebe]

Not completly fixing #342 but greatly reduce the pressure.

At the expanse of losing fractional precision when a latency of second is requested, this PR reduce the code generated by on_xxx function by removing the cardinality of selecting the latency unit in the match expression.

Before the LatencyUnit::Seconds was displayed with xxx.as_secs_f64(), with this patch it is displayed as xxx.as_secs() to be able to cast it to u128 and have an homogenous return type. This allows to remove a cardinality from the huge match we are creating to call the correct log.

before:

❯ cargo bloat --feature trace
0.2%   2.5% 243.4KiB         tower_http <tower_http::trace::on_response::DefaultOnResponse as tower_http::trace::on_response::OnResponse<B,A>>::on_response
0.1%   1.2% 112.6KiB         tower_http <tower_http::trace::on_eos::DefaultOnEos as tower_http::trace::on_eos::OnEos<ReqBody>>::on_eos
0.1%   0.7%  70.4KiB         tower_http <tower_http::trace::on_failure::DefaultOnFailure as tower_http::trace::on_failure::OnFailure<FailureClass,ReqBody>>::on_failure

after this patch:

❯ cargo bloat --feature trace
0.1%   0.7%  64.7KiB         tower_http <tower_http::trace::on_response::DefaultOnResponse as tower_http::trace::on_response::OnResponse<B,A>>::on_response
0.0%   0.3%  30.1KiB         tower_http <tower_http::trace::make_span::DefaultMakeSpan as tower_http::trace::make_span::MakeSpan<B>>::make_span
0.0%   0.3%  30.0KiB         tower_http <tower_http::trace::on_eos::DefaultOnEos as tower_http::trace::on_eos::OnEos<ReqBody>>::on_eos

[jplatte]

Hey @erebe, can you check if #380 gives the same code size gains? That one doesn't change the formatting for the seconds case.

[erebe]

Hello @jplatte,

It is much better than my version, the only bloat that remains with your patch is

0.0%   0.3%  31.2KiB         tower_http <tower_http::trace::make_span::DefaultMakeSpan as tower_http::trace::make_span::MakeSpan<B>>::make_span

the other ones disappeared from the report. 👍

Thank you for having taking a shot at it :)

P.s: I am going into vacation, so may not be super re-active

[erebe]

Closing as #380 is a better solution.