K8SPG-374: detect replication lag on standby

[mayankshah1607]

CHANGE DESCRIPTION

Add support for standby lag detection.

It can be enabled by setting the .spec.standby.maxAcceptableLag field:

apiVersion: pgv2.percona.com/v2
kind: PerconaPGCluster
metadata:
  name: standby-cluster
spec:
  standby:
    enabled: true
    repoName: repo1
    maxAcceptableLag: 10Mi

maxAcceptableLag is the maximum WAL lag allowed for the standby cluster, measured in bytes of WAL data. When this value is exceeded, the cluster primary pod in the standby is marked as unready, the cluster goes into initializing state, and a StandbyLagging condition is set in the status.

CHECKLIST

Jira

• Is the Jira ticket created and referenced properly?
• Does the Jira ticket have the proper statuses for documentation (Needs Doc) and QA (Needs QA)?
• Does the Jira ticket link to the proper milestone (Fix Version field)?

Tests

• Is an E2E test/test case added for the new feature/change?
• Are unit tests added where appropriate?

Config/Logging/Testability

• Are all needed new/changed options added to default YAML files?
• Are all needed new/changed options added to the Helm Chart?
• Did we add proper logging messages for operator actions?
• Did we ensure compatibility with the previous version or cluster upgrade process?
• Does the change support oldest and newest supported PG version?
• Does the change support oldest and newest supported Kubernetes version?

[it-percona-cla]

All committers have signed the CLA.

[nmarukovich]

LGTM but let's fix George's comment

[JNKPercona]

Test Name	Result	Time
backup-enable-disable	passed	00:06:26
builtin-extensions	passed	00:05:19
custom-envs	passed	00:18:10
custom-extensions	passed	00:13:20
custom-tls	passed	00:04:47
database-init-sql	passed	00:02:08
demand-backup	passed	00:23:31
finalizers	passed	00:03:36
init-deploy	passed	00:02:38
monitoring	passed	00:07:20
monitoring-pmm3	passed	00:07:53
one-pod	passed	00:05:44
operator-self-healing	passed	00:09:04
pitr	passed	00:11:59
scaling	passed	00:04:43
scheduled-backup	passed	00:26:05
self-healing	passed	00:09:41
sidecars	passed	00:02:28
standby-pgbackrest	passed	00:11:37
standby-streaming	passed	00:09:10
start-from-backup	passed	00:10:18
tablespaces	passed	00:06:48
telemetry-transfer	passed	00:03:24
upgrade-consistency	passed	00:06:26
upgrade-minor	passed	00:06:32
users	passed	00:04:34

Summary	Value
Tests Run	26/26
Job Duration	01:20:28
Total Test Time	03:43:55

commit: 868df11
image: perconalab/percona-postgresql-operator:PR-1407-868df117a