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/http_metrics/mod.rs

@@ -0,0 +1,149 @@
+//! This crate provides a HTTP server that is solely dedicated to serving the `/metrics` endpoint.
+//!
+//! For other endpoints, see the `http_api` crate.
+pub mod metrics;
+
+use crate::{DutiesService, ValidatorStore};
+use lighthouse_version::version_with_platform;
+use parking_lot::RwLock;
+use serde::{Deserialize, Serialize};
+use slog::{crit, info, Logger};
+use slot_clock::SystemTimeSlotClock;
+use std::future::Future;
+use std::net::{Ipv4Addr, SocketAddr, SocketAddrV4};
+use std::sync::Arc;
+use types::EthSpec;
+use warp::{http::Response, Filter};
+
+#[derive(Debug)]
+pub enum Error {
+    Warp(warp::Error),
+    Other(String),
+}
+
+impl From<warp::Error> for Error {
+    fn from(e: warp::Error) -> Self {
+        Error::Warp(e)
+    }
+}
+
+impl From<String> for Error {
+    fn from(e: String) -> Self {
+        Error::Other(e)
+    }
+}
+
+/// Contains objects which have shared access from inside/outside of the metrics server.
+pub struct Shared<T: EthSpec> {
+    pub validator_store: Option<ValidatorStore<SystemTimeSlotClock, T>>,
+    pub duties_service: Option<DutiesService<SystemTimeSlotClock, T>>,
+    pub genesis_time: Option<u64>,
+}
+
+/// A wrapper around all the items required to spawn the HTTP server.
+///
+/// The server will gracefully handle the case where any fields are `None`.
+pub struct Context<T: EthSpec> {
+    pub config: Config,
+    pub shared: RwLock<Shared<T>>,
+    pub log: Logger,
+}
+
+/// Configuration for the HTTP server.
+#[derive(PartialEq, Debug, Clone, Serialize, Deserialize)]
+pub struct Config {
+    pub enabled: bool,
+    pub listen_addr: Ipv4Addr,
+    pub listen_port: u16,
+    pub allow_origin: Option<String>,
+}
+
+impl Default for Config {
+    fn default() -> Self {
+        Self {
+            enabled: false,
+            listen_addr: Ipv4Addr::new(127, 0, 0, 1),
+            listen_port: 5064,
+            allow_origin: None,
+        }
+    }
+}
+
+/// Creates a server that will serve requests using information from `ctx`.
+///
+/// The server will shut down gracefully when the `shutdown` future resolves.
+///
+/// ## Returns
+///
+/// This function will bind the server to the provided address and then return a tuple of:
+///
+/// - `SocketAddr`: the address that the HTTP server will listen on.
+/// - `Future`: the actual server future that will need to be awaited.
+///
+/// ## Errors
+///
+/// Returns an error if the server is unable to bind or there is another error during
+/// configuration.
+pub fn serve<T: EthSpec>(
+    ctx: Arc<Context<T>>,
+    shutdown: impl Future<Output = ()> + Send + Sync + 'static,
+) -> Result<(SocketAddr, impl Future<Output = ()>), Error> {
+    let config = &ctx.config;
+    let log = ctx.log.clone();
+
+    // Configure CORS.
+    let cors_builder = {
+        let builder = warp::cors()
+            .allow_method("GET")
+            .allow_headers(vec!["Content-Type"]);
+
+        warp_utils::cors::set_builder_origins(
+            builder,
+            config.allow_origin.as_deref(),
+            (config.listen_addr, config.listen_port),
+        )?
+    };
+
+    // Sanity check.
+    if !config.enabled {
+        crit!(log, "Cannot start disabled metrics HTTP server");
+        return Err(Error::Other(
+            "A disabled metrics server should not be started".to_string(),
+        ));
+    }
+
+    let inner_ctx = ctx.clone();
+    let routes = warp::get()
+        .and(warp::path("metrics"))
+        .map(move || inner_ctx.clone())
+        .and_then(|ctx: Arc<Context<T>>| async move {
+            Ok::<_, warp::Rejection>(
+                metrics::gather_prometheus_metrics(&ctx)
+                    .map(|body| Response::builder().status(200).body(body).unwrap())
+                    .unwrap_or_else(|e| {
+                        Response::builder()
+                            .status(500)
+                            .body(format!("Unable to gather metrics: {:?}", e))
+                            .unwrap()
+                    }),
+            )
+        })
+        // Add a `Server` header.
+        .map(|reply| warp::reply::with_header(reply, "Server", &version_with_platform()))
+        .with(cors_builder.build());
+
+    let (listening_socket, server) = warp::serve(routes).try_bind_with_graceful_shutdown(
+        SocketAddrV4::new(config.listen_addr, config.listen_port),
+        async {
+            shutdown.await;
+        },
+    )?;
+
+    info!(
+        log,
+        "Metrics HTTP server started";
+        "listen_address" => listening_socket.to_string(),
+    );
+
+    Ok((listening_socket, server))
+}