Refactor to single published crate

v1.0 publishes two crates: btcnode-metrics and btcnode-prom-metrics. The first gets more attention on crates.io, so probably better for it to be published.

- Make btcnode-metrics a module (non-crate) of btcnode-prom-metrics
- Rename btcnode-prom-metrics to btcnode-metrics
- Publish btcnode-metrics crate & yank btcnode-prom-metrics