Safe cast long to double

[jhaber]

Protobuf has a Struct type which can be used to represent arbitrary JSON structures. However, all numbers are stored as doubles. This means it's easy to accidentally lose precision when sending an integral value. This PR ensures that we only accept integral values if they can be converted to a double without loss of precision.

@stevegutz @kmclarnon @Xcelled

[stevie400]

LGTM, though I worry about existing data causing 500s.

Should we report this to the java protobuf maintainers?

[kmclarnon]

I'm also concerned about just starting to throw in these cases. Can we add an option to enable this behavior and then enable it internally to hubspot?

[jhaber]

It seems like the built-in JsonFormat parsing doesn't throw an exception in this case, eg, this code runs fine and silently loses precision also:

HasValue.Builder builder = HasValue.newBuilder();
JsonFormat.parser().merge("{\"value\":-8747832031878802303}", builder);

I can open an issue on protobuf-java to see whether this is desirable

[jhaber]

protocolbuffers/protobuf#7643