exp: handle : and . in metrics/params

[dberenbaum]

: and . are both special characters for metrics and parameters. Should we disallow them in metrics/params names? Should we enable escaping or some way to specify them?

But my objection is more that this same problem affects every other command/flag combination that uses path:metric_or_param addressing (like exp run --set-param and probably also stage add --params). I would prefer to only have to fix this once, in a consistent and re-usable way. This PR is just patching over one symptom of a broader underlying problem

Originally posted by @pmrowla in #9819 (comment)

Also in the same vein, I'm not sure that we actually document that . is an invalid character in metric or parameter names, but for all intents and purposes we don't support it because we treat . as the dictionary level separator in metric/param addressing (even though it is valid to have string keys that contain . in yaml/json). It would be easier for us to just treat : the same way and disallow it entirely in metric/param names

Originally posted by @pmrowla in #9819 (comment)

[dberenbaum]

Given that it's only come up once AFAIR, setting this to p3.