Telemetry/timestamps and spans

[NelsonVides]

I'm going to want timestamps for all these telemetry events, so that metrics collectors can aggregate and draw plots based on time.

I also want an event for scenario start and stop which will mimic telemetry's span mechanism, but which here it is done in this way because the controller separately calls the beginning and the end of the scenario and span wouldn't fit into this mechanism.

Split from #155

[codecov-commenter]

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[DenysGonchar]

i.e. the exported init/0 function - this is incorrect, it's rather a span between init/1 and terminate/1 call

also I don't think it's a good idea to manually emulate a span here. because scenario doesn't end itself, it finishes per user request. so duration doesn't indicate real scenario execution here. I would rather rework it into 2 events [amoc, scenario, start|stop] with duration reported for stop event. also reporting error during Scenario:termination/1 call as scenario exeption is incorrect. I think it makes sense to report every scenario callback as telemetry span (and we should do it from amoc_scenario module), and amoc_contorller should only issue telemetry events.

[NelsonVides]

Agree to everything except the idea of running Scenario:start/ as a telemetry span in the amoc_scenario module, as this will run in the controller process and measure the time it takes to spawn an erlang process only, instead of measuring the lifespan of the entire amoc user. The current span must stay where it is already and it is the most useful one.

[DenysGonchar]

I'm not sure what you mean here, I propose to measure exactly scenario callback time:
https://github.com/esl/amoc/blob/master/src/amoc_scenario.erl#L64

[DenysGonchar]

well, all of the callbacks.

[NelsonVides]

Oooooh, I see, my bad! Ok, one moment.

[DenysGonchar]

and amoc_scenario module works as a synchronous wrapper for the callbacks. it doesn't spawn anything.

[DenysGonchar]

great PR, thanks for addressing all the comments.