[KIP-714] Metric name from the protocol is read outside boundaries

At
https://github.com/confluentinc/librdkafka/blob/3f52de491f8aae1d71a9a0b3f1c07bfd6df4aec3/src/rdkafka_request.c#L6628
`rd_strdup` is used to duplicate the metric name, but the string isn't NULL terminated, as coming from protocol buffer so it continues reading outside string boundaries.