I suggest you consider inlining this class into InternalSubchannel. MetricsAttributes is awkward, as it isn't at all clear what values are used when. The caller is basically expected to know the implementation here, yet each method has at most two lines. We're not making anything simpler by adding this abstraction. Inlining these would seem to make things much more obvious. It's not like this has tests either; any change here will require a change to InternalSubchannelTest. I'd agree it is disappointing that registering the metrics is so long, but they aren't complicated lines. So complexity is low. (If we needed this API for real, I'd seriously consider having a separate class type passed into each of the methods.)

DisconnectError will end up needing to be public, in order to be used by transports. DisconnectError could be its own file, but really I'd put it in ManagedClientTransport, as it is really a part of the transportShutdown() API. But either way, this doesn't seem like a good place for it, as this class is just a helper for InternalSubchannel.

Thinking about it more, having this utility with just a ton of String arguments is really little different than the metric recorder API itself. In both cases we have to line up the arguments. So we could keep this utility class and just pass in a bunch of arguments instead of trying to package it in MetricsAttributes.

Thinking even more, we can sort of have our cake and eat it too without making a bunch of builders. If use the "lots o arguments" approach, but we put comments in the calling code (InternalSubchannel) for each argument, we can see that the argument names match. And ErrorProne will check that the comments match the variable names in SubchannelMetrics. See https://errorprone.info/bugpattern/ParameterName

(There's still the question of "how much value does the utility provide" and we could still choose to ditch it. But if you do want to keep it, I think this turns out better for this very specific problem at hand.)

The string here is just for debugging. We typically try to match the API where the value is accessible. See ATTR_AUTHORITY_OVERRIDE just above, for example.