Add optional Tracer to appcommon.Config

[leveyjam]

Add the ability to pass an opentracing.Tracer via appcommon.Config.Tracer

We are using harness.go as the basis for the Faro Endpoint, and we would like to configure the OpenTracing->OTel bridge tracer for use in our code ( https://github.com/grafana/app-o11y-kwl-endpoint/blob/main/cmd/kwl/main.go )

@rlankfo authored the 1st commit. I've attempted to add a test to show the behavior is correct.
I will squash the commits before merging if that is preferred?

[CLAassistant]

All committers have signed the CLA.

[ywwg]

Yes, we usually squash commits during merge unless there are very large, disparate changes that make sense to be separated

[ywwg]

thank you for this! I made a few minor style notes, but it looks fine and straightforward

[leveyjam]

Thanks for the swift response @ywwg ! I've accepted your changes + renamed "tearDown" -> "resetTracingGlobals" (I think that explains the purpose of the function and makes it a little bit self-documenting).

[ywwg]

thanks for the quick fixes -- now that I understand the call behavior better, I want to tweak the invocation slightly.

[ywwg]

looks good, thanks!