Create custom tags for all dimensions

[epompeii]

This is a break out task from #127 (comment)

Having key/value pairs could also be useful for the dimensions themselves (branch, testbed, benchmark, metric kind).

Poolifier has very high dimensionality within their benchmarks: https://bencher.dev/perf/poolifier
Being able to easily filter and compare based on tag could be very useful here as well, especially in the Perf Plot.
This also came up in discussions with diesel: diesel-rs/diesel#3849 (comment)