Better handle configuring jaeger tracing with thrift http transport

[chancez]

I'm trying to configure loki to send traces to tempo over the thrift http protocol for for the opentelemetry jaeger receiver.
Loki uses weaveworks/common to set this up.

I've configured my components with the JAEGER_ENDPOINT env var, but I'm getting an error:

020-11-04T14:53:17-08:00 level=error ts=2020-11-04T22:53:17.150980442Z caller=main.go:112 msg="error in initializing tracing. tracing will not be enabled" err="no trace report agent or config server specified"

This appears to come from here:
https://github.com/grafana/loki/blob/master/vendor/github.com/weaveworks/common/tracing/tracing.go#L32-L43

t appears there may be a work-around by specifying some other options, like JAEGER_AGENT_HOST, as that will cause the LocalAgentHostPort configuration to get a default value, bypassing the validation error. And according to https://github.com/grafana/loki/blob/master/vendor/github.com/uber/jaeger-client-go/config/config.go#L416, if the CollectorEndpoint (JAEGER_ENDPOINT) is set, then then the UDP transport isn't even used.

I'm still testing the work around, but https://github.com/grafana/tempo/blob/master/example/docker-compose/docker-compose.loki.yaml#L70-L73 indicates it might work.

[chancez]

As an update, I did get it working when I set JAEGER_AGENT_HOST. I set it to a bogus doesntmatter value and it bypassed the validation error.

I think I'll open a PR to make it check if JAEGER_ENDPOINT is set, as that seems to be the simplest path forward.