Add metrics to VC (#1954)

## Issue Addressed

NA

## Proposed Changes

- Adds a HTTP server to the VC which provides Prometheus metrics.
- Moves the health metrics into the `lighthouse_metrics` crate so it can be shared between BN/VC.
- Sprinkle some metrics around the VC.
- Update the book to indicate that we now have VC metrics.
- Shifts the "waiting for genesis" logic later in the `ProductionValidatorClient::new_from_cli`
  - This is worth attention during the review.

## Additional Info

- ~~`clippy` has some new lints that are failing. I'll deal with that in another PR.~~

common/warp_utils/src/lib.rs

@@ -2,5 +2,6 @@
 //! Lighthouse project. E.g., the `http_api` and `http_metrics` crates.
 
 pub mod cors;
+pub mod metrics;
 pub mod reject;
 pub mod task;