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/Cargo.toml

@@ -16,3 +16,5 @@ safe_arith = { path = "../../consensus/safe_arith" }
 serde = { version = "1.0.116", features = ["derive"] }
 tokio = { version = "0.2.22", features = ["sync"] }
 headers = "0.3.2"
+lighthouse_metrics = { path = "../lighthouse_metrics" }
+lazy_static = "1.4.0"