Introduce monitoring config

[lbradstreet]

Introduce a custom monitoring config that coalesces events and only requires a single send thread per peer-group rather than the new approach in 0.7.10 with a single chosen sender thread per peer

[lbradstreet]

We should be able to generalise the metrics sending with monitoring config as we have done in throughput and latency metrics - though we need to be careful with performance.