Endpoint oddities

[MikeGoldsmith]

    Confirmed setting `HONEYCOMB_API_ENDPOINT` it appends the traces path, and setting `HONEYCOMB_TRACES_ENDPOINT` in another scenario it did not append the traces path or impact `endpoint` or `metricsEndpoint` (👍  matches our spec). Love the tests.

Couple oddities that I think could be addressed in a follow-up issue:

It may be a confusing for a user debugging to see the endpoint being different from tracesEndpoint - tracesEndpoint is what gets chosen over endpoint, and we don't overwrite endpoint value (don't think we should, but is it obvious to an end user which will "win" here?)

  "endpoint": "https://api.honeycomb.io",
  "tracesEndpoint": "http://collector:4318",
  "metricsEndpoint": "https://api.honeycomb.io/v1/metrics",

Another funky thing... When doing the maybeAppendPath for traces and metrics, we check for the existence of v1/traces and v1/metrics for each respectively. If someone sets the generic endpoint and includes v1/traces, their metricsEndpoint is then messed up. Maybe that's okay if we're assuming they only care about traces?

  "endpoint": "https://api.honeycomb.io/v1/traces",
  "tracesEndpoint": "https://api.honeycomb.io/v1/traces",
  "metricsEndpoint": "https://api.honeycomb.io/v1/traces/v1/metrics",

Originally posted by @JamieDanielson in #88 (review)