Add VC metric for primary BN latency (#4051)

## Issue Addressed

NA

## Proposed Changes

In #4024 we added metrics to expose the latency measurements from a VC to each BN. Whilst playing with these new metrics on our infra I realised it would be great to have a single metric to make sure that the primary BN for each VC has a reasonable latency. With the current "metrics for all BNs" it's hard to tell which is the primary.

## Additional Info

NA

validator_client/src/http_metrics/metrics.rs

@@ -191,6 +191,10 @@ lazy_static::lazy_static! {
         "Round-trip latency for a simple API endpoint on each BN",
         &["endpoint"]
     );
+    pub static ref VC_BEACON_NODE_LATENCY_PRIMARY_ENDPOINT: Result<Histogram> = try_create_histogram(
+        "vc_beacon_node_latency_primary_endpoint",
+        "Round-trip latency for the primary BN endpoint",
+    );
 }
 
 pub fn gather_prometheus_metrics<T: EthSpec>(