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.~~

validator_client/src/cli.rs

@@ -138,4 +138,37 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     address of this server (e.g., http://localhost:5062).")
                 .takes_value(true),
         )
+        /* Prometheus metrics HTTP server related arguments */
+        .arg(
+            Arg::with_name("metrics")
+                .long("metrics")
+                .help("Enable the Prometheus metrics HTTP server. Disabled by default.")
+                .takes_value(false),
+        )
+        .arg(
+            Arg::with_name("metrics-address")
+                .long("metrics-address")
+                .value_name("ADDRESS")
+                .help("Set the listen address for the Prometheus metrics HTTP server.")
+                .default_value("127.0.0.1")
+                .takes_value(true),
+        )
+        .arg(
+            Arg::with_name("metrics-port")
+                .long("metrics-port")
+                .value_name("PORT")
+                .help("Set the listen TCP port for the Prometheus metrics HTTP server.")
+                .default_value("5064")
+                .takes_value(true),
+        )
+        .arg(
+            Arg::with_name("metrics-allow-origin")
+                .long("metrics-allow-origin")
+                .value_name("ORIGIN")
+                .help("Set the value of the Access-Control-Allow-Origin response HTTP header. \
+                    Use * to allow any origin (not recommended in production). \
+                    If no value is supplied, the CORS allowed origin is set to the listen \
+                    address of this server (e.g., http://localhost:5064).")
+                .takes_value(true),
+        )
 }