Overhaul stats: Start collecting stats per server instance (per socket)

[josecelano]

Option 1: With independent event sender and stats repositories per server instance

IMPORTANT: This option has been discarded.

When the tracker_usage_statistics is enabled in the HTTP or UDP server config the application should create an independent event sender and metrics repository for that server.

[[udp_trackers]]
bind_address = "0.0.0.0:6969"
tracker_usage_statistics = true 

[[http_trackers]]
bind_address = "0.0.0.0:7070"
tracker_usage_statistics = true

I will implement it by using the option 2 described here.

Option 2. Segregated metrics within the current global stats repositories

More info about this option here.

NOTES

In this issue:

• I will not expose the new metrics via the public REST API yet.
• Metrics are still the same (same metrics and same names as the global metrics).