Add support for overriding hostname and ip fields

[SEJeff]

This mirrors the python and java clients: jaegertracing/jaeger-client-python#167 and jaegertracing/jaeger-client-java#371

C++ isn't a language I'm fluent in, but it looks like this is the relevant code to be changed. I might take a stab at fixing this myself.

The goal is to have the tracing on the the kubernetes ingress controller report the actual host it is running on via a magical env var passed to the pod via the downward api.

[isaachier]

OK cool. Consider using getenv so this can use an environment variable as described here: https://www.jaegertracing.io/docs/client-features/. Not sure if one exists for hostname and/or IP but it would make sense to me. @yurishkuro does that make sense to use an environment variable?

[SEJeff]

That could work but it depends on how it is implemented. Via kubernetes yaml, I can create an env var named NODE_NAME (as an example) with a value of the host running the pod via the downward api. Via that mechanism, I can not however create a variable such as: JAEGER_TAGS with a value of hostname=$NODE_HOSTNAME.

My goal is to make it work (via whatever acceptable means) in the nginx-opentracing library so I can PR it to the ingress for kubernetes.

[yurishkuro]

sounds like there are two enhancements:

1. setting tracer tags via JAEGER_TAGS env variable as supported in other clients
2. bypassing hostname/ip logic in the tracer when hostname/ip tags are given

Don't know about kubernetes, pretty sure it allows setting any kind of env variables, but not sure if it supports substitutions so that one could define JAEGER_TAGS=hostname=$NODE_HOSTNAME

[SEJeff]

@yurishkuro yes that seems sensible. I'm looking for #2 as kubernetes can not do substitutions for env variables provided in the way I need them (via the downwards api).

[rredkovich]

There is relevant issue for nginx jaeger plugin which depends on this feature to be able to be configured with env vars: opentracing-contrib/nginx-opentracing#68

In short: a jaeger agent is placed on every node in kubernetes deployment and to eliminate extra time consumption packets should be send straight to node IP. Node IP where pod is currently deployed could be set dynamically into env var:

        env:
        - name: JAEGER_AGENT_HOST
          valueFrom:
            fieldRef:
              fieldPath: status.hostIP

P.S. Secondly read the thread and second guess - I missed and tags are far from env vars configuration, if so, I'll create a new issue for this, just write so.