Update downstream MySQL doc to reference streaming metrics vs. binlog metrics

The MySQL connector metrics documentation (e.g. at https://debezium.io/documentation/reference/1.6/connectors/mysql.html#mysql-monitoring) still states that the object name of the binlog metrics mbean is debezium.mysql:type=connector-metrics,context=binlog,server=<database.server.name>, but that name is used only if the legacy MySQL connector implementation is used. The object name is debezium.mysql:type=connector-metrics,context=streaming,server=<database.server.name> when using the default MySQL connector implementation in debezium 1.5+.

Annotation comments in mysql.adoc control the how the heading title for this content is presented downstream. These comments must be updated to reflect the change in terminology.