feat(gossip): improved metrics (#276)

improves metrics for inserting gossip values into the gossip table from push messages and pull requests

this pr also includes a few bug fixes:
- grafana: now uses the correct delta on metrics (was prev using incorrect idelta)
- table: prev a full table was not allowing updating of existing labels, this changes it
- table: when gossip data was updated, we were not free-ing the old overwritten data, this fixes it
- cmd: fixes the predefined mainnet entrypoints
- pull_requests: dont send requests to peers which have a different shred version than us