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.~~

Cargo.lock

@@ -7382,8 +7382,10 @@ dependencies = [
  "futures 0.3.8",
  "hex",
  "hyper 0.13.9",
+ "lazy_static",
  "libc",
  "libsecp256k1",
+ "lighthouse_metrics",
  "lighthouse_version",
  "logging",
  "parking_lot 0.11.1",
@@ -7535,6 +7537,8 @@ dependencies = [
  "beacon_chain",
  "eth2",
  "headers",
+ "lazy_static",
+ "lighthouse_metrics",
  "safe_arith",
  "serde",
  "state_processing",