What does this PR do?

1. Adds DDR (Disaster Recovery) support to the synthetics_private_locations resource, enabling PL replication from a source org (R1) to a destination org (R2).

Related dogweb PRs:

• Add PL metadata related to DDR
• Support PL DDR metadata in PL endpoints (create)
• Update Get Private Location with include_pl_info

RFC: https://docs.google.com/document/d/1pxwa2vqa5I_NkhWlhXlcuFsDgSbBVQNdeXFEQ1vzQSM/edit?tab=t.0#heading=h.tmwe8zpx1a8q

2. Migrates synthetics_tests off skip_resource_mapping=True to use the map_existing_resources infrastructure. Adds a cross-run dedup short-circuit so a sync run that finds a destination test already stamped with a known source_public_id adopts it into state instead of POSTing a duplicate. This guards against orphan accumulation when prior sync POSTs succeeded server-side but the client never received the response (504-after-success under destination API load).

Description of the Change

New CLI option: --datadog-host-override

• Added to constants.py (DD_DATADOG_HOST_OVERRIDE env var), options.py (CLI flag), and configuration.py (threaded to the Configuration dataclass).
• Optional CNAME override passed to the DDR create endpoint for DNS failover.

Updated synthetics_private_locations.py:

1. excluded_attributes: added ddr_metadata, pl_id, and public_key_test. ddr_metadata is returned on the destination for DDR PLs and must not be diffed against the source. pl_id and public_key_test are returned by the source-side GET (with include_pl_info=true) and are only used at create time, so they must not show up as diffs.

2. import_resource(): calls GET /api/v1/synthetics/private-locations/{id}?include_pl_info=true so the source state captures pl_id and public_key_test at import time. No extra fetch at sync time.

3. create_resource(): when a source PL is being created at the destination:

   • Reads pl_id + public_key_test from the source state (already captured at import).
   • Strips null metadata from the request body (DDR endpoint rejects it).
   • Injects ddr_metadata.disaster_recovery with source_pl_id and source_name. The dogweb schema is strict (additionalProperties: false) and accepts only these two fields.
   • Sets test_encryption_public_key to the JSON-stringified public_key_test object ({pem, fingerprint, id}).
   • Optionally sets datadog_host_override.
   • Parses the DDR response and returns resp["private_location"]. When --datadog-host-override is set, it appears at private_location.config.datadogHostOverride and ships in the regular destination state JSON.

Updated synthetics_tests.py:

1. resource_config: replaced skip_resource_mapping=True with resource_mapping_key="metadata.disaster_recovery.source_public_id". That stamp is already injected on every source test by pre_resource_action_hook, so it doubles as the dedup key.

2. map_existing_resources() override: fetches destination tests via GET /api/v1/synthetics/tests?include_metadata=true and indexes them by source_public_id. Custom override avoids the destination-versions side-effect that the existing get_resources() has.

3. create_resource() dedup short-circuit: before issuing a new POST, checks if a destination test already carries this source_public_id. If yes, adopts it into destination state and calls update_resource() instead of POSTing. Prevents new duplicates when re-syncing after a prior orphan-creating run.

Test fixture update: moved synthetics_tests from OPT_OUT_RESOURCES to MAPPING_RESOURCES in tests/unit/test_map_existing_resources.py.

Operational guidance

When running against staging (app.datad0g.com), throttle concurrency to avoid McNulty pool exhaustion (512 status codes; see Confluence):

--max-workers 20 --http-client-retry-timeout 180

The default --max-workers 100 saturates dogweb's gunicorn pool on staging, producing 512s and 504s. The 504s are particularly bad on POST because dogweb may have written the resource server-side before timing out, and the client's retry then duplicates it. Throttling keeps the pool happy and eliminates this in-POST duplication path. Prod has more headroom and can usually run at the default.

Known limitations

In-POST retry duplication: if a single POST attempt 504s and the client retries, dogweb may have already created the resource. Each retry creates another server-side row. The throttling guidance above is the operational mitigation; a proper fix (skipping 504 retries on POST in custom_client.py) is out of scope for this PR.

Verification Process

Tested end-to-end on staging (app.datad0g.com) between two orgs with --max-workers 20 --http-client-retry-timeout 180:

1. Created PLs and tests in source org (R1).
2. Ran import: confirmed source state captured pl_id and public_key_test via include_pl_info=true, and that every R1 test landed in resources/source/synthetics_tests.json.
3. Ran sync: confirmed PLs created via the DDR endpoint with the right schema, tests created in R2 with metadata.disaster_recovery stamps, and destination state matches R2 counts after throttled runs.
4. Verified end-to-end DDR replication: manually unpaused one of the replicated tests in R2 and confirmed synthetics_private_locations_check_assignment populated and the test began running on the replicated PL.
5. Ran a second sync after dropping a destination state entry, and confirmed the dedup short-circuit adopts the existing R2 test instead of creating a duplicate.

Follow-up

A separate PR will revisit the reset command semantics ("delete only sync-cli-managed resources" vs current "delete everything in R2"). That conversation is out of scope here.

Release Notes

• Added DDR (Disaster Recovery) support for synthetics_private_locations.
• New --datadog-host-override CLI option for optional CNAME override during PL replication.
• Migrated synthetics_tests to map_existing_resources with cross-run dedup against metadata.disaster_recovery.source_public_id. Re-running sync after a degraded run no longer accumulates duplicate tests in the destination org.