Update to prometheus-client in libp2p-metrics v0.21.1

Needs prometheus/client_rust#137.

Author: mxinden

misc/metrics/Cargo.toml

@@ -28,7 +28,7 @@ libp2p-ping = { workspace = true, optional = true }
 libp2p-relay =  { workspace = true, optional = true }
 libp2p-swarm = { workspace = true }
 libp2p-identity = { workspace = true }
-prometheus-client = { version = "0.21.0" }
+prometheus-client = { version = "0.21.1"}
 once_cell = "1.16.0"
 
 [target.'cfg(not(target_os = "unknown"))'.dependencies]