[sql] Include instance name in db.namespace

[alanwest]

The MSSQL conventions require db.namespace to be qualified by the instance name.

[3]: When connected to a default instance, db.namespace SHOULD be set to the name of the database. When connected to a named instance, db.namespace SHOULD be set to the combination of instance and database name following the {instance_name}.{database_name} pattern.

Currently, the instance name is parsed from the data source string and captured as the deprecated db.mssql.instance_name attribute.

Including the instance name in the db.namespace is required, so this may influence the conversation on #2235.