Add support for `update` calls

[MondayHopscotch]

An update would find and update the value of a queued metric. This would be very useful for values that might change very quickly and we don't want to send many values over the wire.

[MondayHopscotch]

I think a better abstraction here is to separate "events" and "gauge" metrics.

Events are one-time and should always be reported as queued.

Gauges (term borrowed from stats) are reported on an interval, and only the latest value is actually sent as a data point. This is important /useful for frequently changing data where all we want is the latest value.

Examples of each of these are:

Events:
Player death, new game started, item "ultra sword" attained

Gauges:
Current speed, max depth (holey moley), money in wallet