remove interface label for interface_stats

[huntergregory]

To reduce metric cardinality, we should aggregate interfaces for interface_stats. We filter out several interfaces, and only error/drop stats are included, which are unlikely, but technically, there is risk that the number of interface names could increase over time.

Before:

interface_stats{interface=eth0, stat=...}

After:

interface_stats{stat=...}

[anubhabMajumdar]

Since this metric has been out for sometime, should we keep the interface label and set that to a constant value like allInterfaces ? Otherwise we may break existing tooling that interfaces with this metric (like Grafana dashboards).