Refactor snapshots collector (prometheus-community#789)

- Move metric Desc to vars to aid in unused linter checks
- Use new Collector interface
- Add a util getURL func. Similar funcs are in most of the collectors. The only difference is typically what struct to unmarshal the JSON into. This func just returns []byte so that the caller can handle the unique data structure.

This does not adjust the labels on the metrics. I want that to be handled separately to make sure the refactor itself matches existing behavior.

Signed-off-by: Joe Adams <[email protected]>

Author: sysadmind

CHANGELOG.md

@@ -1,3 +1,11 @@
+## master / unreleased
+
+BREAKING CHANGES:
+
+The flag `--es.snapshots` has been renamed to `--collector.snapshots`.
+
+* [CHANGE] Rename --es.snapshots to --collector.snapshots #XXX
+
 ## 1.6.0 / 2023-06-22
 
 BREAKING CHANGES: