Capture Request Queue Time

[cschneid]

Load balancers timestamp incoming requests into one of several headers. It indicates the amount of time spent waiting for an available server, and is good for determining if web servers are over or under provisioned.

We need to capture that, find the diff from "now" and record that time.

Record it as a RequestTag, with name: scout.queue_time_ns, and value of the integer number of nanoseconds. Get whatever accuracy is available, and then convert to nanoseconds.

The necessary CoreAgent code to capture this is not yet written, but will be ready soon.

See also the python code: https://github.com/scoutapp/scout_apm_python/blob/b7a9b9a9d2d028906c0ec6e5058ee9b56cb11533/src/scout_apm/core/queue_time.py#L37

See also the ruby code:
https://github.com/scoutapp/scout_apm_ruby/blob/master/lib/scout_apm/layer_converters/request_queue_time_converter.rb