Collect usage metrics

[pulkitsinghal]

As of now, I don't think that Vend collects API usage metrics. It would be neat to:

1. collect API usage metrics, based on the number of times a method of the underlying vend-nodejs-sdk is invoked
2. collect metrics on which commands are used most often

This data can stay on the user's own machine and should only be shared upstream, in an aggregated and anonymous manner ... when a user explicitly decides to share it ... by toggling the shareMyUsageMetrics flag in settings.json to true

[pulkitsinghal]

A new line can be added to usageMetrics.json file each time to track a usage event. This should help if a user decides to share metrics after X amount of time after using vend-tools (say a month?) and should make it simple to continue sharing in a semi-live fashion moving forward.

We will probably need some kind of proxy with DB credentials that can avoid DDOS and still tack on authN headers to persist the data in a backend.