Parsing fails if the response value is NaN

[seriouszyx]

github.com/prometheus/client_golang v1.16.0
go version: go1.20.4

If the response is NaN when querying metrics, the following error will occur:

{"error": {"external":"bad_response: readObjectStart: expect { or n, but found 1, error found in #1 byte of ...|1:78: parse|..., bigger context ...|1:78: parse error: unexpected identifier \"xxx\" in l|..."}