Use singular form for directory names

[rhcarvalho]

It is common practice in Go to name package path components in singular, so

• exporter/metric/...
• propagator
• detector
• instrumentation (currently already in singular form)

Right now of the 4 areas of this repo, one is already in singular form and the other 3 are plural.

Other Go repos in OpenTelemetry do follow the practice of using singular, for example the Collector and Collector-Contrib.

Opening this issue to discuss renaming exporters/, propagators/ and detectors/ to the singular form before GA.

[MrAlias]

Changing these names needs to be done with a lot of caution. It will change the module/package names meaning there will be discoverability issues in the future when users find the old deadend modules/packages and think they are the current ones.

There are examples of plurals in other Go projects including the standard library: bytes, errors, strings. We should be sure we are making the change for an agreed upon reason instead of a preference here.

[MrAlias]

Thanks for the suggestion. At this point the stability of the packages this provides outweighs the desire to use more common singular names in the package. Closing.