Pull out cockroach-metrics, add minimal stats to inventory (#8426)

This PR directly follows the extraction from
#8379

It pulls two of these metrics into inventory, where they will be used by
the reconfigurator to decide if a Cockroach zone can be safely updated
in #8441.

Author: smklein

Cargo.lock

@@ -9,6 +9,8 @@ anyhow.workspace = true
 chrono.workspace = true
 cockroach-admin-client.workspace = true
 futures.workspace = true
+parallel-task-set.workspace = true
+reqwest.workspace = true
 serde_json.workspace = true
 slog.workspace = true
 serde.workspace = true