util: change some layers to require recorders that are Sync

[tobz]

Context

Mentioned in #533, our recent change to require the global recorder to be Sync had a downstream effect for users taking advantage of certain layers like Fanout and Router. This is due to the fact that those layers only require taking something that is Recorder, which then is held as a Box<dyn Recorder>.

Solution

This PR updates Fanout and Router (and by extension, their builders, FanoutBuilder and RouterBuilder) to take recorders that are Sync, and then holds them as Box<dyn Recorder + Sync>. This facilitates being able to once again use these layers as part of the global recorder.

The original intent, before the change to require Sync in the global recorder, was always to have layers be usable for the global recorder, so this change is simply aligning these utilities to match.

While there are areas where a Sync recorder is not required, such as metrics::with_local_recorder or metrics::set_default_local_recorder... they're much more rare and generally scoped to testing, where something like DebuggingRecorder is being used and there would be no reason to use a layer. If somehow it turns out that there's a need for non-Sync layers for these use cases, we can address this by creating Unsync variants in the future.

[tobz]

Released as metrics-util@v0.19.0.