Add number of matched gaia sources in statistics.yaml

[rgcosentino]

Just taking a previous field in the statistics section and adding this. RTB requests this new stat at some point after commissioning heading into operations.

number_matched_gaia_sources:
title: Number of Matched Gaia Sources
description: |
The number of detected sources in the exposure that have been
successfully matched to sources in the Gaia catalog.
type: number
sdf:
special_processing: VALUE_REQUIRED
source:
origin: TBD
archive_catalog:
datatype: int
destination: [WFIExposure.number_matched_gaia_sources]

[schlafly]

The number of matched sources is currently in the wcs_fit_results object that is populated after tweakreg runs. It's not in the schema and doesn't go to the archive but is in the files, FWIW. Does this change your sense for whether we need to get it in?

And, if not, I think it makes more sense for us to propagate some subset of the wcs_fit_results to the archive rather than put this in schema. Can you look at the current wcs_fit_results values and decide what you would like propagated from there?

[rgcosentino]

Let me take to RTB and get feedback. It's not urgent for commissioning.