A json containing a string field value with the '\n', when converted to protobuf and back results in invalid json with line breaks.

The following escaping should be done:

'"'  => "\\\"";
'\\' => "\\\\";
'\t' => "\\t";
'\b' => "\\b";
'\n' => "\\n";
'\r' => "\\r";
'\f' => "\\f";

Source: RFC 7159:

All Unicode characters may be placed within the
quotation marks, except for the characters that must be escaped:
quotation mark, reverse solidus, and the control characters (U+0000
through U+001F).