fix: URLs in OTLP exporters

[TommyCpp]

Fixes #1618
Design discussion issue (if applicable) #

Changes

• Clearify the documentation that the endpoint in ExporterConfig should be complete. i.e exporter will not append signal path onto it.

• Removed the signal path for default grpc endpoints. As in grpc protocol, path is decided by the package name. Users can still add path if they manually config it

Path is case-sensitive. Some gRPC implementations may allow the Path format shown above to be overridden, but this functionality is strongly discouraged. gRPC does not go out of its way to break users that are using this kind of override, but we do not actively support it, and some functionality (e.g., service config support) will not work when the path is not of the form shown above.

• Don't append anything if the users add a path in ExportConfig. URLs from env vars don't append anything so we should keep them consistent

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[TommyCpp]

block on #1704 for the new version fix