This commit adds an abstraction that provides a standard manner to
retrieve various metadata that are shared by most data sources.

DataSourceMetadata is implemented by the three data source
implementations that boot supports out-of-the-box: Tomcat, Hikari and
Commons dbcp.

This abstraction is used to provide two additional metrics per data
source defined in the application: the number of allocated
connection(s) (.active) and the current usage of the connection pool
(.usage).

All such metrics share the 'datasource.' prefix. The prefix is further
qualified for each data source:

* If the data source is the primary data source (that is either the
  only available data source or the one flagged @Primary amongst the
  existing ones), the prefix is "datasource.primary"
* If the data source bean name ends with "dataSource", the prefix is
  the name of the bean without it (i.e. batchDataSource becomes batch)
* In all other cases, the name of the bean is used

It is possible to override part or all of those defaults by
registering a bean with a customized version of
DataSourcePublicMetrics.

Additional DataSourceMetadata implementations for other data source
types can be added very easily, check
DataourceMetadataProvidersConfiguration for more details.

Fixes gh-1013