Create histogram metrics for SLIs

[pboothe]

Create histograms for:

• time spent by savers in unknown state
• amount of data buffered before being "opened". (maybe - how do you poll a channel for its buffer usage?)

Using the definitions:

• p0 - time PH first seeing a flow
• t0 - time tcpinfo sees a flow (Timestamp of event)
• p1 - time PH gets open signal from tcpinfo.

Create histograms for:

• p0-t0 Note that avg(p0 - t0) is expected to be a small constant on the order of .01 seconds
• p1-t0 Note that avg(p1 - t0) expected to be much less than 1 sec.

The averages/medians/90th percentiles seem like sensitive SLIs.