Update metrics collector to track readiness
In some cases it may take a lot of time between when the instance provisioned by fleeting enters the running
state and when it's really ready to be used by taskscaler. This may change further depending on what taskscaler's caller wants to do.
This MR adds a metric that tracks timings of the instance becoming ready (counted from when it's discovered to be in the running
state). Including the optional caller's upFunc()
execution.
Edited by Tomasz Maczukin