Turn on the logger's interest cache

[koute]

polkadot companion: paritytech/polkadot#5393

This lowers the CPU overhead of our logging machinery when any trace log is enabled down to ~3% of what it is without any trace logs enabled.

Background

The tracing-log crate has a bunch of relatively expensive filtering machinery which decides whether a given log should be printed out or not. This machinery has a fast path: if the current most verbose logging level is set to X then the logger can just reject every log which uses a more verbose level and skip the expensive checks.

For example, if we have set RUST_LOG to foo=info,bar=warn then the most verbose log level is info, so any trace logs can be quickly rejected because we know that a log must be at least info to be printable. Now, if we have set RUST_LOG to foo=info,bar=trace then the most verbose log level is trace, so now for any trace log the logger must actually check whether that log came from foo or from bar to decide whether it should be printed out, and this is expensive when you're generating thousands of trace logs every second (even if they're not printed out!).

So this PR enables a built-in interest cache which I've previously added to tracing-log. This cache allows the logger to essentially skip the heavyweight filtering machinery by just doing the work once, and then just querying the cache on any subsequent call. According to the measurements I've done a long time ago this should cut down the CPU usage of the logger to ~3% when you have any trace log enabled (even if it's for a target which doesn't actually exist and will never actually match anything).

(This is the second time I'm making this PR; the previous PR couldn't be merged since the tracing-log crate was not released on crates.io; but that finally changed today with the release of tracing-log 0.1.3.)

[gilescope]

Great to see it land at last! Good things come to those that wait.

[koute]

bot merge

[paritytech-processbot]

Error: "Check reviews" status is not passing for paritytech/polkadot#5393

[koute]

bot merge