Decomposing the layer

[beanow-at-crabnebula]

Hey there :]
I'm seeing some great stuff in this library. Though I think it doesn't compose nicely at the moment.

I would like to add the metric tracking layer and the views, without the global setup and export route.

Currently the library does everything with one layer.

• Creating your global MeterProvider and Registry
• Adding views to the MeterProvider
• Set up a Meter and tracking axum metrics
• Adding a /metrics route to export on

Ideally we can pick-and-choose from this.

Rough outline of what that could be.

The views don't take any arguments, so could be a static method or it's own builder, say ViewBuilder::default().build()

The layer would exclusively measure, and it's builder could have an optional MeterProvider using the global one if None.

The export router would be a separate builder. Which could either take the current approach (make a new MeterProvider & Registry pair and set it as global) or be created from arguments passed in.

[ttys3]

Very good advice.
It's just that I don't have enough time to do this refactoring at the moment. PR is welcome -_-

Also, I think besides pull mode prometheus exporter,
We also need to support other exporters, such as otlp.
Under this design, the prometheus specific /metrics endpoint should be an optional feature.