Allow custom Registry on HttpMetricsLayerBuilder

[kennedyoliveira]

Adding option to allow setting a custom prometheus registry, so it can hook into other places using prometheus directly.

[ttys3]

Thank you for your PR.

I am currently refactoring this crate. Since the otel rust no longer recommends using the pull mode exporter, I am planning to remove the built-in convenient support from this crate. However, users can still add the Prometheus export by themselves. This approach is more flexible and allows the crate itself to focus more on its own functionality.

[ttys3]

since v0.9.1, you can use a custom prom registry now.
please see https://docs.rs/axum-otel-metrics/0.9.1/axum_otel_metrics/index.html#advanced-usage