bugsnag: emit slice_name from service_name in custom metadata tab

[morgan-wowk]

Some Bugsnag stacks filter on custom.slice_name to assign errors to
a slice. Store the service_name passed to setup() and write it as
custom.slice_name on every event so errors from orchestrator, api, and
metrics_poller each land in their own slice.

Consolidates all custom tab writes into a single add_tab call.

[morgan-wowk]

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• bugsnag: propagate cloud_provider into contextual logging for queued executions #268
• bugsnag: emit slice_name from service_name in custom metadata tab #267 👈 (View in Graphite)
• bugsnag: chain-aware grouping key and chain title in error list #262
• bugsnag: strip JSON objects from grouping strings, normalize LauncherError #261
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[Mbeaulne]

🤖 This is an AI-generated code review comment.

setup() docstring should mention the new slice_name side-effect.

service_name used to be only the SDK's project_root. After this PR it is also stored as module state and surfaced on every event as custom.slice_name. Callers reading the existing docstring won't know that picking the service_name string also affects Bugsnag custom metadata on every event going forward.

Fix: one-line addition to the Args docstring:

service_name: Identifies the process in Bugsnag (e.g. "tangle-api").
              Also surfaced as `slice_name` in the "custom" metadata tab on every event.

[Mbeaulne]

🤖 This is an AI-generated code review comment.

Silent key collision if operator sets TANGLE_BUGSNAG_CUSTOM_GROUPING_KEY=slice_name.

custom: dict[str, str] = {}
if _service_name:
    custom["slice_name"] = _service_name
...
custom[_CUSTOM_GROUPING_KEY] = key_value   # could overwrite "slice_name"

_CUSTOM_GROUPING_KEY is operator-supplied via env var. If someone sets it to slice_name — perhaps not realizing the new key exists — the grouping value will silently overwrite the slice name and the tab will only contain one of them.

Fix: low-effort options:

• Warn at module load: if _CUSTOM_GROUPING_KEY == "slice_name": _logger.warning(...) once, so the operator finds out.
• Or prefix Tangle-owned custom keys (tangle_slice_name) to make collisions almost impossible.
• Or just document the reserved key in the env-var comment block at the top of the file.

[Mbeaulne]

🤖 This is an AI-generated code review comment.

Missing negative-path test for service_name=None / no grouping key.

The new code has two paths: (a) _service_name set → emit slice_name, (b) _service_name None AND no grouping key → skip the entire add_tab("custom", ...) call thanks to the if custom: guard at the bottom of _before_notify. Path (a) is tested by this test; path (b) isn't pinned anywhere. A future refactor could silently start emitting an empty custom tab on every event without any test failing.

Fix: add a sibling test:

def test_before_notify_omits_custom_tab_when_no_service_name_and_no_grouping_key(monkeypatch):
    monkeypatch.setenv("TANGLE_BUGSNAG_API_KEY", "test-api-key")
    monkeypatch.delenv("TANGLE_BUGSNAG_CUSTOM_GROUPING_KEY", raising=False)
    importlib.reload(bugsnag_module)
    contextual_logging.clear_context_metadata()
    with mock.patch("bugsnag.configure"), mock.patch("bugsnag.before_notify"):
        bugsnag_module.setup(service_name=None)

    mock_event = mock.MagicMock()
    mock_event.original_error = None
    mock_event.metadata = {}
    bugsnag_module._before_notify(mock_event)

    custom_calls = [c for c in mock_event.add_tab.call_args_list if c.args[0] == "custom"]
    assert custom_calls == []