put v1/traces as constant

[tigroo]

We need to open an issue on this as for today this parameters defaults to /v1/traces but it as so requires the end user to also set /v1/traces when customizing the endpoint.

/v1/traces is actually some kind of a constant, so it should not be read from the configuration but hardcoded by the telemetry init function. And here we should only set the endpoint path if the target OTLP collector is routed so.

(this will also align implementation with the python one and maybe others)

Originally posted by @nbuffon in #417 (comment)