[jaeger exporter] Add env var for agent host

[naseemkullah]

Is your feature request related to a problem? Please describe.
To point to an agent daemonset pod running on local node, the value has to be set in the kubernetes spec as v1:status.hostIP (this cannot be set in code, not easily at least) , it would be very convenient to simply add a JAEGER_AGENT_HOST env var that the exporter looks for without it being explicitly set in the exporter config, and if it is not set, default to localhost of course. please see `open-telemetry/opentelemetry-specification#172 (comment)

Describe the solution you'd like
JAEGER_AGENT_HOST or JAEGER_HOST whose value will be used as the host in the exporter config, if set.

Describe alternatives you've considered
adding process.env.JAEGER_AGENT_HOST in all projects that use open telemetry with jaeger exporter.

Additional context

status.hostIP - the node’s IP, available since v1.7.0-alpha.1

https://kubernetes.io/docs/tasks/inject-data-application/downward-api-volume-expose-pod-information/#capabilities-of-the-downward-api

[mayurkale22]

Should we wait for specs issues to get approved/merged?

[dyladan]

This is really an SDK mechanism, but at the same time I think this should be decided on for all SIGs, not just ours. I'm not sure where questions that are not spec related but affect all SIGs should go. Probably still in spec.

[dyladan]

@mayurkale22 maybe we should consider some sort of waiting-on-specs label for issues like this?

[dyladan]

Or we could close it and re-open when the spec is ready.

[mayurkale22]

@mayurkale22 maybe we should consider some sort of waiting-on-specs label for issues like this?

I like this idea, created a label "waiting-for-spec".

[naseemkullah]

This can be closed as we went ahead with JAEGER_AGENT_HOST via #924