metrics endpoint: restrict to relevant server metrics

[macfarla]

// TODO Each app server has a /metrics endpoint but currently each endpoint returns the metrics
// for all servers.  Would be better to lock this down e.g. <p2puri>/metrics only returns the
// p2p metrics

https://github.com/ConsenSys/tessera/blob/095ab4fee41d13bb914b10b366ab8ecc6c6018da/server/jersey-server/src/main/java/com/quorum/tessera/server/monitoring/MetricsResource.java#L30