Add OC-Agent Node Exporter.

[songy23]

The OC-Agent Node exporter should:

1. Act as a client of the Trace RPC service of OC-Agent;
2. Grab current Spans from OpenCensus-Node library, transform them to Span proto, and apply the incoming Config from Agent.

For reference, please see OC-Agent Go Exporter and OC-Agent Python Exporter.

/cc @justindsmith

[justindsmith]

Thanks @songy23, I’ll start taking a look at this over the next week or so. The only glaring issue I see right now is that the node sampler doesn’t support rate sampling, but it looks like the go oc-agent exporter treats that config change as a no-op right now, so I’ll just do the same until we can get that work done.

How is testing being done with the oc-agent today? Is there a way for me to validate my changes with a running agent instance?

There are also some other concerns architecturally around an exporter modifying the tracer config, but there are a few options that shouldn’t be too disruptive, so I’ll play around with that.

[songy23]

Thanks for the help with the Agent Exporter :)

The only glaring issue I see right now is that the node sampler doesn’t support rate sampling, but it looks like the go oc-agent exporter treats that config change as a no-op right now, so I’ll just do the same until we can get that work done.

Yes, actually no languages have support for rate-limiting sampler at this moment since it's just proposed to be added to our spec (census-instrumentation/opencensus-specs#158). For now it's safe to just ignore it.

How is testing being done with the oc-agent today?

In OC-Agent-Go-Exporter we're testing the Agent exporter against a mock Agent. Please take a look at mock_agent_test.go.

Is there a way for me to validate my changes with a running agent instance?

Unfortunately testing against a real Agent instance is not available at this moment. @odeke-em and I are actively working to add Agent impl to OC-Service repo, please expect that to be done in the following week.

There are also some other concerns architecturally around an exporter modifying the tracer config, but there are a few options that shouldn’t be too disruptive, so I’ll play around with that.

Exactly, that's also why we don't 100% commit to the config update feature with Agent:

We MAY also give Agent the ability to push configurations (e.g sampling probability) to Library.

[songy23]

Fixed in #128.