Test our doc: getting_started folder

[zhihali]

Try to start work #4001, would like to start from getting started file: https://github.com/open-telemetry/opentelemetry-python/tree/be02f980f3eb2e43f8afd9c586c4d3c02e7d5906/docs/getting_started

Currently, this folder have 4 examples
flask_example.py
metrics_example.py
otcollector_example.py
tracing_example.py

But the test only have
test_flask.py and test_tracing.py

I think to keep the consistency, we should have the metrics and otcollector in test as well, even though these 2 are pretty easy example.

Not sure if my understanding correct, if it make sense, will open a PR for it.

[zhihali]

Just some confusions:

I found most of docs are under opentelemetry.io repo, and I think if we google the otel-python, the link will point me to opentelemetry.io doc as well, it's good and include many contents https://opentelemetry.io/docs/languages/python/
And, in our otel-python repo, it include a doc here: https://opentelemetry-python.readthedocs.io/en/latest/index.html. This doc include some API and SDK, and some examples. It's totally different from the otel io one. And also, this one is a bit strange. For example:

1. in our otel-python repo, it have some getting start code(docs/getting_started), but in this link it doesn't include these codes. I can only find these test in a really old version doc, the repo is not belong to Otel : https://opentelemetry-python-kinvolk.readthedocs.io/en/latest/getting-started.html
   and the repo is: https://github.com/kinvolk/opentelemetry-python/blob/master/docs/getting-started.rst
   seems this one is phased out? If so, we should not include these tests in our CI anymore, we should clean it.

2. in our otel-python(https://opentelemetry-python.readthedocs.io/en/latest/index.html)
   it include some examples, but it looks unfinished
   
   like this one, it just have no examples
   
   Shall we start doing these? or just not a priority? or should delete this.

3. Codes show in otel.io include some from our repo(like: docs/examples), how these be synced and tested?
   idea from Emídio:

We have two docs and they are for different purposes. [otel.io](http://otel.io/) is for users, read the docs is for developers
If we find a way to have the examples in the otel.io rendered from our repo, maybe using gist or something like that we can have control we are testing everything in both sides

[pmcollins]

Personally I don't know much about readthedocs.io, but since it's a third party site, should it be secondary priority? The opentelemetry.io one we own and can change though: https://github.com/open-telemetry/opentelemetry.io