InfluxDB: Backend migration validation

[itsmylife]

We recently reverted the backend migration feature toggle #64825
The removal was causing problems. We also discovered some inconsistencies between frontend parsing and backend parsing. Before removing frontend access we should reach parity.

Previous job has been done for backend migration:

• Remove the migration feature toggle from InfluxDB #60321
• InfluxDB migration to backend data source #43076

Tasks:

• Gather all issues related to feature toggle removal
• Define pain points in those issues and create sub-tasks to address them https://github.com/grafana/observability-metrics-squad/issues/123
• Modify diff-checker to use for InfluxDB
• Check the responses and fix issues InfluxDB: Backend parser compatibility with frontend parser #69865
• Make sure the retention policy is working as expected InfluxDB: Return default retention policy from backend as first element #69818
• Address all the pain points that were gathered in the first step
• Introduce instrumentation with RED Metrics
• Have a dashboard in place with useful metrics to show anomalies
• Have alerting rules to warn us about anomalies
• Use phasing while rolling it out. More context about phasing
• Make the backend mode the default mode in a minor version v10.3.0
• Remove the feature toggle in the next minor version v10.4.0
• Check the performance of InfluxDB Client

[itsmylife]

Thread for related issues https://raintank-corp.slack.com/archives/C04UFN2HDS5/p1678896173312019

[itsmylife]

Dashboard for the feature toggle: https://ops.grafana-ops.net/d/48INDsjnk/grafana-feature-toggle-prr?orgId=1&var-feature=influxdbBackendMigration&var-slug=ismailsimsek&from=now-3h&to=now

Test dashboard to see the feature toggle in the wild: https://ismailsimsek.grafana.net/d/bd1e61e1-3d5d-40e2-994e-dde0b3c20c0a/gc-duration-seconds-sum?orgId=1&var-retention=autogen&var-tagKeysInfluxInfo=commit&var-tagValForVersion=go1.20.1&tab=transform&from=now-24h&to=now

[itsmylife]

Switching to backend mode is breaking the table visualization panels with field transformations or field overrides
Issue: #76897
Fix PR: #76899