Tracking Issue for Attributes being Experimental.

In #1700 it was decided that Attributes is still experimental. This issue is acts the the reference from the source code.

[ejona86]

API review:

• Key.of(debugString) is actually a bit weird; we were originally doing Key.of(name) which isn’t as weird. Across all similar key types (Context, Attributes, and this one), we want to swap to the form Key.create(debugString) and Key.createWithDefault(debugString, T default). So that would mean adding create here and deprecating of().
• Consider removing keys(); need to investigate what it is being used for. We’re fine with it, but it is a bit weird; we’re not sure what it’s useful for. We are okay with keys() here because Attributes was intended to be an immutable (and type-safe) map and is only really passed between the producer and consumer. Unlike Context and CallOptions, there doesn’t seem much point to visibility-restricting certain keys (so you can’t access the value if you can’t get at the instance), which is the main thing having keys() would defeat. But because each key is semantically quite distinct, it is hard to determine what someone is using it for.
• Replace static newBuilder(Attributes) with non-static toBuilder(). Deprecate (and later remove) newBuilder(Attributes)

With those tweaks, looks good to stabilize.

[ejona86]

If this was done on purpose then I suppose that there should be another way to get attribute by name.

The thing is that attribute keys don’t have a name. The string passed is a debug string. We could maybe add some more language to Attributes.Key to at least mention “uses reference equality” like we do for CallOptions.Key or fuller text like we have in Context.Key (although that full text requires keys() to be removed).

Attributes is generic and used in multiple different circumstances. The ones listed in Grpc may not be exhaustive. A specific transport can have its own, for example. But it does seem we could add a breadcrumb in the {Client,Server}Call.getAttributes() javadoc to point to Grpc as the source of some of the common attributes. We should change the debug string to also be the API name for the key; if you saw the debug string as io.grpc.Grpc.TRANSPORT_ATTR_REMOTE_ADDR then the way to use it becomes obvious.