Opentelemetry-java: Provide usage examples

[arminru]

We should add samples for using OpenTelemetry Java to make it easier for beginners to get a feeling of how to use the API properly.
OpenCensus had examples included right in the repo and so do OTel Python, JS, Go and .NET.

[thisthat]

I can take care of it!

[arminru]

Should we make the examples a subproject which breaks the build if the API is changed without updating the examples accordingly?

Having the examples as standalone projects which reference OTel via Maven makes more sense for users looking at them but this way they're more likely to get outdated if no one cares to update them regularly.

[carlosalberto]

Should we make the examples a subproject which breaks the build if the API is changed without updating the examples accordingly?

I personally like this, as otherwise it would be very easy to forget about them ;)

[bogdandrutu]

I think we have the first set of examples, we should file more granular issues if we need more examples.