Merge custom resources on detection

[c24t]

This PR changes resource detection (monitored_resource.get_instance) to include custom type and resource labels if available, and to include labels from multiple resources if we detect more than one known environment (e.g. kubernetes on AWS).

Fixes #457 and #514.

cc @mayurkale22

[songy23]

I suggest we fix #510 first, since it doesn't make much sense to combine gke_containter resource with AWS resource (while it makes sense for k8s_container to also be an EC2 instance).

[reyang]

@c24t I'd like to hear about your opinion.

Shall we consider the signature of os.path.join? https://docs.python.org/2/library/os.path.html#os.path.join

[c24t]

Part of the motivation for making the arg a list was to match the signature in java, but since *args in python is more idiomatic than varargs in java, maybe this is the better approach here, and I like this pattern as a user.

You think callers are more likely to have to make a list to call merge_resources than have to splat a list if we mimic os.path.join?

[c24t]

I put up https://github.com/c24t/opencensus-python/pull/1/files#diff-21856523a57af46b9f02f815944713e3R89 to see what this would look like.

[reyang]

I don't have strong opinion here, just putting another option and see which one you would prefer.
Please do what you think is better. Either way looks good to me :)

[c24t]

@songy23 good call, let's wait on this until after #510.

[reyang]

LGTM, let's wait for #510.