Defines the HTTP configuration for an API service. It contains a list of
[HttpRule][google.api.HttpRule], each specifying the mapping of an RPC method
to one or more HTTP REST API methods.
An indicator of the behavior of a given field (for example, that a field
is required in requests, or given as output but ignored as input).
This does not change the behavior in protocol buffers itself; it only
denotes the behavior and may affect how API tooling handles the field.