Metrics links duplicate anchor IDs

Now that the user doc for the metrics tables have clickable links, the anchor IDs for the links will cause duplicate ID conflicts when trying to build the user guide downstream. The metrics clickable links are not in the upstream 1.2 branch so this is not a problem for the Q3 release. 

For the next downstream release, the way the metrics anchor IDs are now, they will prevent the downstream user guide from building.  The duplicate ID errors happen because the doc for each connector includes the same metrics content. This duplicates the anchor ID in the doc for each connector that has that metric.

To resolve this, we will add a variable to each metrics link. The variable will have a name such as "dbtype" and the value will be set in the doc for each connector, e.g., "db2", "mysql", "postgresql", "sqlserver", "mongodb".