Add additional validator set metrics

[aaronbuchwald]

Why this should be merged

This PR adds a total weight metric to the peer tracker.

How this works

The peer tracker already tracks the total weight in order to calculate the percent connected, so this change adds an additional function to the interface to expose TotalWeight and adds a metric called total_weight into tracked peers.

How this was tested

CI + running locally to get the updated metric:

avalanche_P_total_weight 4e+16
# TYPE avalanche_P_num_validators gauge
avalanche_P_num_validators 20

[StephenButtolph]

I think we can get rid of the TotalValidators function - it doesn't seem needed.