[proxysql] Generate metric for all pairs of hostgroup/status (backend)

[frivoire]

What does this PR do?

Ensure that the count metric is always generated, even when 0 backend exists in that status & hostgroup at the moment.

Motivation

Today, when a hostgroup has no online backend server for example, the metric with tag status=ONLINE+hostgroup=42 is not generated at all (no value for sometime, until a backend is online).
But having no online backend is an important piece of information to monitor a proxysql.
Example: a dashboard showing the nb of online backend servers per hostgroup is not giving a good observability if not showing the 0 value, because it has a different meaning than no value :(

NB: the list of hostgroups is dynamic (because it depends on config, thus different for each deployment), and the list of status is hardcoded (because fixed in proxysql, cf doc)

Tests

I've only tested the SQL query alone on a real proxysql:

Before:

ProxySQLadm> SELECT hostgroup, status, COUNT(*)
FROM stats_mysql_connection_pool GROUP BY hostgroup, status;
+-----------+--------------+----------+
| hostgroup | status       | COUNT(*) |
+-----------+--------------+----------+
| 10        | ONLINE       | 1        |
| 10        | SHUNNED      | 2        |
| 20        | ONLINE       | 3        |
| 30        | ONLINE       | 2        |
| 9999      | OFFLINE_HARD | 3        |
+-----------+--------------+----------+

Now:

ProxySQLadm> SELECT refpairs.hostgroup, refpairs.status, count(stats.srv_host) as count
             FROM (
                 SELECT DISTINCT(hostgroup), refstatuses.status
                 FROM stats_mysql_connection_pool
                 JOIN (
                     select 'ONLINE' as status union all select 'SHUNNED' as status union all
                     select 'OFFLINE_SOFT' as status union all select 'OFFLINE_HARD' as status
                 ) refstatuses
             ) refpairs
             LEFT JOIN stats_mysql_connection_pool stats ON stats.hostgroup=refpairs.hostgroup and stats.status=refpairs.status
             GROUP BY refpairs.hostgroup, refpairs.status;
+-----------+--------------+-------+
| hostgroup | status       | count |
+-----------+--------------+-------+
| 10        | OFFLINE_HARD | 0     |
| 10        | OFFLINE_SOFT | 0     |
| 10        | ONLINE       | 1     |
| 10        | SHUNNED      | 2     |
| 20        | OFFLINE_HARD | 0     |
| 20        | OFFLINE_SOFT | 0     |
| 20        | ONLINE       | 3     |
| 20        | SHUNNED      | 0     |
| 30        | OFFLINE_HARD | 0     |
| 30        | OFFLINE_SOFT | 0     |
| 30        | ONLINE       | 2     |
| 30        | SHUNNED      | 0     |
| 9999      | OFFLINE_HARD | 3     |
| 9999      | OFFLINE_SOFT | 0     |
| 9999      | ONLINE       | 0     |
| 9999      | SHUNNED      | 0     |
+-----------+--------------+-------+

=> same values than before (when not zero) ✅
=> and all other pairs of hostgroup+status are now also generated as 0 🆕

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[frivoire]

NB: I've also notified the Datadog support about this issue => request #1997802

[HadhemiDD]

1 - can you add the changelog entry for this change:
ddev release changelog new

2 - can you fix the linter:
ddev test proxysql --fmt

[HadhemiDD]

Change requested.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.74%. Comparing base (6782bb7) to head (ac604ed).
Report is 28 commits behind head on master.

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