Need an option to prevent add ",omit_empty" for json label

[sparkthu]

When the json is used to communicate with browser, all field with omit_empty will cause lots of missing for keys.

[puellanivis]

Was not sure about how this aligned with the standards, so I looked it up:

An implementation may provide options to emit fields with default values in the JSON-encoded output.

https://developers.google.com/protocol-buffers/docs/proto3#json

It appears to be a valid option to do. But maybe the option name could be narrowed to be clear that it is json-specific: json_emit_default_values.

[dsnet]

The generation of the json tag predates the finalization of the JSON<->Protobuf specification. When that spec was finalized, it became clear that the json tag alone could not be used to implement the specification (unless the encoding/json provides some advanced features). In retrospect, I wish we never generated the json tags.

That being said, adding a command-line option to remove this is not the right solution for several reasons:

• It's an option that provides fine-grained control of a niche part of the generated output and is not generalized to other use-cases
• We need to think carefully about adding any more commandline flags as that may interact poorly if we ever do proposal: protoc-gen-go should be able to compile multiple packages per run #39, which would allow protoc-gen-go to output multiple files at once. In such a case, would the commandline flag affect all outputted files? If yes, is that wise to do?
• It's use case becomes fuzzy if we ever do protoc-gen-go: generate MarshalJSON/UnmarshalJSON methods for messages #256, which would subvert control of the json tags.
• I'd rather see protoc-gen-go: support go_tag option to specify custom struct tags #52 be supported, which can be used to specify this yourself.

I'm going to close this in favor of #52, which is the closest issue to being able to support this.