improve libp2p connected peer metrics (#4870)

* improve libp2p connected peer metrics

* separate discv5 port from libp2p for NAT open

* use metric family for DISCOVERY_BYTES

* Merge branch 'unstable' of https://github.com/sigp/lighthouse into improve-metrics

common/lighthouse_metrics/src/lib.rs

@@ -244,6 +244,16 @@ pub fn inc_counter_vec(int_counter_vec: &Result<IntCounterVec>, name: &[&str]) {
     }
 }
 
+/// Sets the `int_counter_vec` with the given `name` to the `amount`,
+/// should only be called with `ammount`s equal or above the current value
+/// as per Prometheus spec, the `counter` type should only go up.
+pub fn set_counter_vec_by(int_counter_vec: &Result<IntCounterVec>, name: &[&str], amount: u64) {
+    if let Some(counter) = get_int_counter(int_counter_vec, name) {
+        counter.reset();
+        counter.inc_by(amount);
+    }
+}
+
 pub fn inc_counter_vec_by(int_counter_vec: &Result<IntCounterVec>, name: &[&str], amount: u64) {
     if let Some(counter) = get_int_counter(int_counter_vec, name) {
         counter.inc_by(amount);