Add a metric label template mechanism

The API makes an effort to create unique metric labels when comparing data
across multiple runs, but without human direction the constructed names aren't
necessarily ideal to help a human focus on the comparisons they want to make.

This PR adds a UI template mechanism to allow the human to format names that
are more useful -- for example, to focus on the differences between software
releases or hardware configurations expressed by Crucible tags or params.

For example, a template of `<metric> <iteration>:<period> <tag:accelerator>`
identifies metrics between two runs based on the hardware accelerator type
used for each.

This introduces a new action to fetch and store the available filters, which
exposes the param and tag values currently in use. Usability of the template
mechanism will depend on consistent application of specific tags, which we
expect to be increasingly supplied automatically by Crucible discovery rather
than relying on the (current) ad hoc user definition in endpoint files.