attributes: values must be comparable (not documented)

[benpbrown]

Since #4855, attribute values must either implement Equal(o interface{}) bool or be comparable.

This previously wasn't the case and this restriction isn't documented as far as I can tell.

https://pkg.go.dev/google.golang.org/grpc/attributes mentions that keys must be hashable, but does not mention that values must be comparable [if they don't implement Equal]. That information can maybe be derived from If Equal is not implemented, standard equality is used to determine if the two values are equal., but I think it could be more clear.

I would prefer that it was possible to use un-comparable standard types (e.g. using https://pkg.go.dev/github.com/google/go-cmp/cmp as the fallback comparator), but if that isn't possible, I think there should be some documentation indicating directly that values need to be comparable.

We saw this panic in services after a gRPC version bump.

What version of gRPC are you using?

grpc@v1.43.0

What version of Go are you using (go version)?

Go playground

What operating system (Linux, Windows, …) and version?

Go playground

What did you do?

Executed https://go.dev/play/p/TYD55tjljGj

What did you expect to see?

True or false

What did you see instead?

A panic

panic: runtime error: comparing uncomparable type map[string]string

goroutine 1 [running]:
google.golang.org/grpc/attributes.(*Attributes).Equal(0x4887e0, 0xc00009acc0)
	/tmp/gopath3674506896/pkg/mod/google.golang.org/grpc@v1.43.0/attributes/attributes.go:93 +0x194
main.main()
	/tmp/sandbox3370463311/prog.go:16 +0x207

Program exited.

[menghanl]

We cannot use the cmp package because it's intended to only be used in tests, and it panics (e.g. for structs with unexported fields).

We will update the doc to add that all values must be comparable.