Reimplement ManagedIdentityCredential with composition

[chlowell]

ManagedIdentityCredential exists to hide MsiCredential and ImdsCredential behind a facade that has the right surface for documentation and code completion. It accomplishes this by overriding its constructor to return an instance of one of those classes, and defining empty methods with docstrings.

It could instead hold another credential and wrap its methods, as does EnvironmentCredential.

[HankiGreed]

I want to take this issue for my first contribution. Would appreciate if someone could give me a little headstart about it.

[chlowell]

The gist is that ManagedIdentityCredential is currently a factory that can't itself be instantiated. It overrides __new__ to return one of the implementation credentials. So, you call ManagedIdentityCredential() and get something else:

>>> ManagedIdentityCredential()
<azure.identity._credentials.managed_identity.ImdsCredential object at 0x011EAF88>

ManagedIdentityCredential could instead have an ordinary __init__ and hold an instance of the appropriate implementation credential. It could implement get_token by wrapping that credential's get_token method, similarly to EnvironmentCredential. I suggest starting with a look at EnvironmentCredential.

[chlowell]

Addressed by #9302, thanks @HankiGreed!

[HankiGreed]

Thanks for giving me a chance to do this. This is my first contribution on GitHub and it felt good doing it. Thanks @chlowell for bearing with me throughout. !