ui: expose txn.refreshspanbytesexceeded in the admin UI

[irfansharif]

In #46056 we added this metric to track when queries fail to refresh reads due to having run into the limit set by kv.transaction_max_refresh_span_bytes. We expect more txns to hit this limit as explained in #46275. We should expose this metric in the admin dashboards, coupled with guidance around how to work around it in our docs (cockroachdb/docs#7012), to help users running into serializable retry errors.

Jira issue: CRDB-5068

[github-actions]

We have marked this issue as stale because it has been inactive for
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to CockroachDB!

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
10 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!