Add render metrics for partials and templates

Subscribe to action_view's events for rendering templates and partials.

This metric deviates from others in that the source is the template/partial being rendered, in the following format:

    <view_dir>[:<view_dir]:<template>

For example:

    users:create.html.erb
    sessions:destroy.html.haml
    ...

All metrics are reported as sporadic by source.

Still mulling over whether I'm completely happy with this. On a related note perhaps cache metrics should use the cache key as a source in a similar fashion?