maint: Don’t append OTLP signal paths if already present

[MikeGoldsmith]

Which problem is this PR solving?

If an HTTP endpoint for traces or metrics already includes the signal path (eg /v1/traces or /v1/metrics), the path is added a second time (eg http://somewhere.com/v1/traces becomes http://somewhere.com/v1/traces/v1/traces).

The path should only be added if it doesn't already exist.

• Closes When creating HoneycombOptions, do not append /v1/traces to traces_endpoint if it's already there #155

Short description of the changes

• Update options.get_traces_endpoint and options.get_metrics_endpoint to check if the endpoint already ends with the signal path before adding it
• Add tests to verify behaviour

How to verify that this has the expected result

You can now use an endpoint with a signal path and the signal path won't be added twice.