"Query": @"insert into MetersTest (a,b,c,d) values (@a,@b,@c,@d)"
JSON files don't share C#'s "verbatim string
]" construct. Strings must be delimited with double quotes, with no prefix, and any special characters within the string must be properly escaped.
Since there are no special characters in the string you're using, you just need to remove the
prefix from the string literal:
"Query": "insert into MetersTest (a,b,c,d) values (@a,@b,@c,@d)"
The JSON in your "What I have tried"
section is valid JSON. Since you haven't explained what's wrong it, we can't tell you how to fix it.