chore(datadog_metrics sink): avoid sending an interval for non-rate counters

[tobz]

Currently, the sink calculates an interval as the number of seconds since we last generated an encoded payload and applies that to counters which do not have their own interval.

While this doesn't seem to actually affect the display of these counters when graphed, given that they're marked as counts, and not rates... it's still confusing that we calculate and apply this, given that it seems to have no effect.

We could simplify the code in that area by avoiding constructing an interval (that ultimately does nothing) and only using one if the metric itself has one defined.