Add more null channels for metrics_clients_last_seen test

[scholtzan]

Some more error data for evaluating the observability tools

Checklist for reviewer:

• [ ] Commits should reference a bug or github issue, if relevant (if a bug is referenced, the pull request should include the bug number in the title).
• [ ] If the PR comes from a fork, trigger integration CI tests by running the Push to upstream workflow and provide the <username>:<branch> of the fork as parameter. The parameter will also show up in the logs of the manual-trigger-required-for-fork CI task together with more detailed instructions.
• [ ] If adding a new field to a query, ensure that the schema and dependent downstream schemas have been updated.
• [ ] When adding a new derived dataset, ensure that data is not available already (fully or partially) and recommend extending an existing dataset in favor of creating new ones. Data can be available in the bigquery-etl repository, looker-hub or in looker-spoke-default.

For modifications to schemas in restricted namespaces (see CODEOWNERS):

• [ ] Follow the change control procedure

┆Issue is synchronized with this Jira Task

[dataops-ci-bot]

Integration report for "Add more null channels for metrics_clients_last_seen test"

sql.diff

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```diff diff -bur --no-dereference --new-file /tmp/workspace/main-generated-sql/sql/data-observability-dev/fenix_derived/metrics_clients_last_seen_v1/query.sql /tmp/workspace/generated-sql/sql/data-observability-dev/fenix_derived/metrics_clients_last_seen_v1/query.sql --- /tmp/workspace/main-generated-sql/sql/data-observability-dev/fenix_derived/metrics_clients_last_seen_v1/query.sql 2024-05-27 17:24:24.000000000 +0000 +++ /tmp/workspace/generated-sql/sql/data-observability-dev/fenix_derived/metrics_clients_last_seen_v1/query.sql 2024-05-27 17:24:23.000000000 +0000 @@ -15,13 +15,12 @@ AND submission_date = "2024-05-23" AND sample_id < 50 UNION ALL --- create an entry where normalized channel is null +-- create entries where normalized channel is null SELECT * REPLACE (NULL AS normalized_channel), FROM `moz-fx-data-shared-prod.fenix_derived.metrics_clients_last_seen_v1` WHERE submission_date = @submission_date - AND submission_date = "2024-05-24" -LIMIT - 1 + AND submission_date = "2024-05-27" + AND sample_id = 0 ```

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