Background:
When a bot advertises dimension values, they are returned ordered.

When a value is created out of split items, like done in issue 762161, where 'zone': ['us', 'us-central', 'us-central1', 'us-central1-c'], the first value is the most broad/less informative one.

Actual:
- Zone column displays the less precise item, 'us'.

Expected:
- Zone column displays the most precise item, 'us-central1-c'.

AI:
- Figure out a way to detect this case and:
  - Skip the display of common prefixes
  - Display values that are not a prefix of others

_pool_from_dimensions() has similar algorithm.
Ref: https://cs.chromium.org/chromium/infra/luci/appengine/swarming/ts_mon_metrics.py?l=259