Add convenience metric

We rely on grafana, prometheus, and alertmanager for our monitoring stack. When metrics are ingested, they contain a non-human-friendly logid such as `entries_added{logid="abcdef1234567890abc"}`. In the [docs](https://github.com/google/trillian/blob/master/docs/api.md) there is an optional display name that can be set per shard. It would be a big help if a metric could be exported that contained the `display_name`, `tree_state`, and `tree_type`.

A metric such as this would be perfect `shard_information{logid="abcdef1234567890abc",display_name="2020",tree_state="active",tree_type="LOG"}`.

With this proposed metric, we would be able to use Prometheus' [group_left](https://www.robustperception.io/using-group_left-to-calculate-label-proportions) and make the rest of our metrics human-friendly.

Here's an example of what we currently do in grafana. Configuring this across multiple dashboards is time consuming, requires manual intervention when adding a new shard, and is prone to error. Additionally, because this is done in grafana, the human-friendly name can't be sent to a prometheus alert.

I've considered writing a database exporter to generate a metric, but I think it would be better suited to be built into trillian instead.

Perhaps this already exists and I've missed it. If not, thank you for considering it.