fix: px protocol decode - do not treat missing response field as error

[NagyZoltanPeter]

Description

With v0.33.0-rc2 on status.staging it was obvious that client using PeerExchange RPC the old way - on request they completely leave out response field (not even default is added) - cause decode error on nwaku side - that now implements changed protocol.

Changes

• PeerExchangeRpc will not handle missing response field as error.
• By adding Peer Exchange metrics to grafana dashboard it is better to use counters instead of gauge for better dashboard stats.

• rpc_codec of WakePeerExchange
• waku_px_peers_sent and waku_px_errors have changed to counter from gauge type.

Issue

#3054

[github-actions]

You can find the image built from this PR at

quay.io/wakuorg/nwaku-pr:3055

Built from f1644f9

[gabrielmer]

Thank you!