DM-26456: Create sky object performance metric trackers

[leeskelvin]

No description provided.

[laurenam]

Are all of skyObject, skyLevel and skyMean really useful here (and below, skyMean in the Std definition)?

And, again, I think it may be a better idea to leave the specific flux algorithm from the name and you could indicate in the description that the particular flux measurement algorithm is included in the metadata.

[leeskelvin]

I've reduced the tags to just - skyObject and - skyLevel for both the sky object mean and standard deviation metric measures.

(PS - Following conversations we've had over the prior couple of weeks and the issues surrounding shortening these metric names, I've left the metric names as-is for now)

[leeskelvin]

PPS: As a small update for the sake of accuracy, I've updated the two description strings to mention the fact that these are statistical values based on sigma-clipped data, i.e.:

The mean value of the sigma-clipped sky object base_CircularApertureFlux_9_0_instFlux data.
The standard deviation of the sigma-clipped sky object base_CircularApertureFlux_9_0_instFlux data.