Add support for importing counters to Stackdriver

[knyar]

Currently query results get written to Stackdriver as GAUGE metrics.

This does not work well for importing SLIs, since an SLI is typically a ratio of two counters (good_events / valid_events). To allow calculating a ratio over an arbitrary long time window, we'll need ts-bridge to import both counters (good_events and valid_events) to Stackdriver separately as cumulative metrics.

This is not trivial, since CUMULATIVE metrics in Stackdriver require the client to report start_time (metric reset time) with each point. This timestamp is usually set to the start time of the process exporting a counter, and is used by Stackdriver to track counter resets.

I believe we should be able to use Datadog's cumsum function to get a cumulative counter, and then export it to Stackdriver as a CUMULATIVE metric.

This will require keeping some internal state per counter, and I think we can use metric records for that.