fix(seer): Use count(value) in tracemetrics query for metric metadata

[isaacwang-sentry]

Fix the get_metric_metadata RPC handler to use count(value) instead of zero-arg count() in its tracemetrics query.

The tracemetrics dataset parser requires count() to take an attribute argument. Zero-arg count() parse-fails at the events layer with status=400 body={'detail': 'Invalid number of arguments for count, was expecting 1 arguments', 'code': 'parse_error'}. Every call to this handler since it was merged in #113462 has been returning events_query_failed — the Seer assisted-query metrics agent has never once received real candidates from this tool and has been silently falling back to get_field_values probes instead.

The bug wasn't caught because the existing tests mocked sentry.api.client wholesale and never exercised the real parser. This PR also adds TestGetMetricMetadataIntegration, which persists real TraceItems and hits the handler end-to-end via TraceMetricsTestCase — this is the shape of test that would have caught the original bug, and will catch future regressions in the aggregate-function shape.

While here, the ApiError catch block now attaches status_code and a 500-char body prefix to its logger.exception extras so future flakes are debuggable from logs without a new deploy cycle.

Reproduction (before this PR, on prod):

POST /api/0/organizations/1/seer-rpc/get_metric_metadata/
{"args": {"org_id": 1, "project_ids": [1], "name_substrings": ["storage.put.size"], "stats_period": "7d", "limit": 20}}

returns {"candidates": [], "has_more": false, "error": "events_query_failed"} on every substring, 25/25 tries. After this PR the same request returns populated candidates.

Refs #113462