Add taped metric for session state

Now that we have some insights in the read/write speeds from telemetry, we need to be able to correlate this with the session state. E.g. we are seeing a lot of 0 read/writes, but this is (most likely) because the drive is not actively transferring.

Metrics are not really designed for emitting categories, but we should be able to do this easily with a gauge:

• Gauge emits an integer
• The integer corresponds to a given session state

Will update this ticket with more details