fix(spans_metrics): fall back to update on 409 during create

[riyazsh]

Summary

spans_metrics sets skip_resource_mapping=True, so state.destination["spans_metrics"] is never populated from a live LIST. First-run against a destination that already has a same-named metric (customer-created before sync-cli started managing the org) POSTs, gets HTTP 409 "metric already exists with that name", and loops forever — state stays empty, next dispatch retries the CREATE, same 409.

The v2 API keys spans_metrics by name (URL path param metric_id == metric name), so a 409 on POST means the same _id is on the destination. This PR makes create_resource catch 409, GET the existing record by id, hydrate state.destination, and delegate to update_resource (PATCH). Non-409 errors propagate unchanged.

Motivation

Observed in production against a real customer org where 18/18 spans_metrics failed every sync run with HTTP 409. The destination org had all 18 metric names pre-created by the customer (managed=false); sync-cli had no way to converge because it never saw them.

The same class of defect affects any resource type that combines:

• skip_resource_mapping=True (no live LIST populating state)
• create_resource doing a bare POST with no 409 fallback
• A resource "id" that is a human-chosen name rather than a server-assigned UUID

I only fix spans_metrics here; other candidates (metric_percentiles, metrics_metadata, name-keyed order/config types) should be evaluated individually.

Test plan

• pytest tests/unit/test_spans_metrics_409_fallback.py -v → 3 passed
  • test_create_resource_happy_path — POST succeeds, no fallback triggered
  • test_create_resource_409_falls_back_to_get_then_patch — POST 409 → GET-by-id → state.destination hydrated → PATCH called with id-scoped URL
  • test_create_resource_non_409_reraises — 500 propagates unchanged, no GET/PATCH
• Existing spans_metrics behavior unchanged on the happy path (same return shape, same POST call)

Files changed

• datadog_sync/model/spans_metrics.py — wrap create_resource POST in try/except; on 409, GET-by-id, hydrate state, delegate to update_resource
• tests/unit/test_spans_metrics_409_fallback.py — new unit tests