Add initializr specific metrics

This commit adds several project related metrics that are recorded
using the standard CounterService.

The following metrics are managed:

* `counter.initializr.requests` sums the total number of requests, be
it for a project or a simple build file
* `counter.initializr.dependency.xyz` represents the number of times
the xyz dependency was requested. If the dependency is requested
through an alias, it is consolidated using the standard id
* `counter.initializr.type.xyz` represents the statistics per project
type (i.e. starter.zip, pom.xml, etc)
* `counter.initializr.java_version.xyz` represents the statistics per
java version
* `counter.initializr.packaging.xyz` represents the statistics per
packaging (war, jar)
* `counter.initializr.language.xyz` represents the statistics per
language (java, groovy)
* `counter.initializr.boot_version.xyz` represents the statistics per
Spring Boot version

The statistics are recorded by default using a
ProjectGenerationListener implementation. This can be further
customized by overriding the default ProjectGenerator bean provided
through auto configuration.

Fixes gh-32