Handle trailing slash on endpoint for otlphttp

[cartermp]

The current otlphttpexporter doesn't handle when there's already a trailing slash in the configured endpoint.

The spec says that this kind of stuff should be handled w.r.t. the OTEL_EXPORTER_OTLP_ENDPOINT env var and says nothing about programmatic handling of this config. But I still think it's a worthwhile change on a common-ish[1] ergonomic issue.

[1] while the collector isn't the only source of this, at honeycomb we emitted an error related to a misconfigured otlphttp endpoint ~2.5 million times over the past month