[Bug] Clean up metrics feature flag

[vicsn]

🐛 Bug Report

The metrics feature is enabled by default, so the fact that there exists a metrics feature is confusing.
diff.txt.

Additionally, even if the metrics are disabled (see appended diff), the metrics endpoint is still accessible.

This cleanup is not high priority by itself, but it is key that features are handled uniformly so that we don't mess up implementing consensus-critical features like test_targets

Expected Behavior

Ideally:

1. The top-level Cargo.toml has metrics in its default feature list, which can be used to enable or disable all metrics functionality
2. The metrics endpoint is not available if the metrics feature is not enabled.