Add an authority_discovery_known_authorities_count metric (#6614)

substrate/client/authority-discovery/src/addr_cache.rs

@@ -68,6 +68,11 @@ where
 		self.cache.insert(id, addresses);
 	}
 
+	/// Returns the number of authority IDs in the cache.
+	pub fn num_ids(&self) -> usize {
+		self.cache.len()
+	}
+
 	// Each node should connect to a subset of all authorities. In order to prevent hot spots, this
 	// selection is based on randomness. Selecting randomly each time we alter the address cache
 	// would result in connection churn. To reduce this churn a node generates a seed on startup and