Gauge min-max values should be settable via min/max queries

__Proposal:__
Gauge min/max values are only hard-coded. This means the dashboard creator has to guess at what good min or max values are. If we could set the gauge max value to be "the max value we've seen", this could be much more useful as an indicator of whether we are performing at max capacity.

__Current behavior:__
values are hard-coded

__Desired behavior:__
values settable via query or via max/min value.

__Alternatives considered:__
none.

__Use case:__
I'm using it for a metric throughput gauge. the max value is going to be different for everyone.

The issue names no files or tests. Start by tracing dashboard gauge configuration and query handling, then clarify whether min/max values come from query results or observed extrema; done means gauge limits can be supplied dynamically for the throughput use case.

Written by the indexing model from the issue text.