Display name validation is misleading

[nilebox]

There are a few issues with the display name validation:

1. Display name must be prefixed with opentelemetry/ according to the regex

   cloud-operations-api-mock/internal/validation/mock_metric_validation.go

   Line 48 in e8bcc34

   displayNameRegex = regexp.MustCompile(`opentelemetry/.*`)

   Why do we check for this? This is not documented in go/ot-gcp-requirements

2. When display name doesn't match the regex mentioned above, mock server returns the error "displayName has max length of 128 bytes" (see

   cloud-operations-api-mock/internal/validation/statuses.go

   Lines 40 to 41 in 7216ea7

   	statusInvalidDisplayName = status.Error(codes.InvalidArgument,
   	fmt.Sprintf("displayName has max length of %v bytes", maxDisplayNameBytes))

   ), which is super misleading: we don't even check for the string length, and should have specified in the error that we expected it to be prefixed with opentelemetry/ or something more user friendly.

[aabmass]

I noticed the same