#274 added a required `impactedTestsEnabled` field to the SDK's SettingsResponse
model but didn't update the integration-test mock backend, so the settings
response it returned no longer decoded — failing every integration test that
boots the SDK against the mock.

Emit `impacted_tests_enabled` from MockBackend.buildSettingsResponse (configurable
via Settings.impactedTestsEnabled, default false). Matches the field set the SDK
decoder requires, as covered by LibraryConfigurationServiceThrowTests.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* Replace OTel metrics SDK with a custom telemetry collector

Telemetry distributions are sent to the DD intake as raw sample arrays so the
backend computes the summary. The OTel-histogram path bucketed samples and then
reconstructed approximate values from bucket midpoints on export — lossy
(distorts percentiles) and complex (separate explicit/exponential code paths).

Drop the OpenTelemetry metrics SDK (meter provider, views, periodic reader, the
TelemetryMetricExporter bridge) in favor of a small lock-protected MetricStore:
counters accumulate a per-interval delta, distributions keep raw samples, and
each drain takes-and-resets the buffers (delta semantics). The store is drained
to the durable TelemetryExporter via a new TelemetryPayloadExporter sink
protocol.

Two robustness refinements:
- A short flush interval (10s) shrinks the window of in-memory metrics a crash
  could lose — export() persists to disk immediately and upload happens later on
  the exporter's own schedule, so this adds no network traffic.
- When buffered distribution samples reach a cap, record() force-flushes so a
  burst is persisted rather than dropped (dd-trace-go drops on a full buffer).

The metric tree and tag enums are unchanged; only the Counter/Distribution/
Factory handle internals now target the store. Tests rewritten against an
in-memory payload sink; ARCHITECTURE.md updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* Make telemetry flush interval and distribution buffer size configurable

Wire the two collector tunables to Config so they can be overridden by env vars,
mirroring DD_TELEMETRY_HEARTBEAT_INTERVAL:
- DD_TELEMETRY_FLUSH_INTERVAL (seconds, clamped 1...3600, default 10)
- DD_TELEMETRY_DISTRIBUTION_BUFFER_SIZE (samples, clamped >= 1, default 2048)

DDTracer threads them from Config into Telemetry; the hardcoded defaults remain
as the fallback for direct construction.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* Key telemetry metrics by a tag Set instead of a sorted array

Set equality/hashing is intrinsically order-independent, so the metric key
no longer relies on remembering to sort tags at record time, and duplicate
"key:value" entries collapse. The wire array is sorted once per series at emit
time for deterministic output. Matches dd-trace-go semantics (same name +
different tags = separate series).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* Guard MetricStore state with Synced instead of a bare UnfairLock

Wraps the counters/distributions/sample-count in a single Synced<State>,
matching the locking pattern used elsewhere in the SDK (e.g. DDModule) rather
than hand-rolling an UnfairLock. Behavior is unchanged: take-and-reset on flush
via `defer { s = State() }`.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* Raise default distribution buffer cap to 65536

The cap is a global safety backstop for bursts; the 10s timer does the normal
draining. Samples are 8-byte Doubles, so a generous cap is cheap (65536 ≈ 512 KB)
and keeps force-flush from triggering under normal CI telemetry volume.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* Stamp telemetry metric points with the SDK clock, not Date()

MetricStore now takes the NTP-synced SDK clock (DDTestMonitor.clock) and uses
clock.now for count-series timestamps instead of a raw Date(), matching the
rest of the SDK's time handling. Tests pass a DateClock().

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* Remove unused telemetry namespace properties from Resource

telemetryMetricNamespace / telemetryDistributionNamespace were only read by the
deleted OTel TelemetryMetricExporter bridge. The custom collector stamps the
.civisibility namespace directly on each payload, so these Resource accessors
have no remaining readers.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* Merge duplicated metric types in the telemetry tree

Most metrics differed only in name, not shape: an untagged counter, an untagged
distribution, or a counter tagged solely by error_type / event_type / endpoint.
Replace the ~30 near-identical leaf structs with six reusable handle types
(NoTagCounter, NoTagDistribution, ErrorTypeCounter, EventTypeCounter,
EndpointCounter, EndpointDistribution) plus matching Factory helpers.

Metrics with a unique tag signature (events.created/finished, session.started,
git.command*, settings_response, requests_errors, code_coverage library
counter) keep their own struct. Call sites and wire output are unchanged;
TelemetryMetrics.swift drops from 682 to 513 lines.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* Add missing impacted_tests_enabled to mock backend settings response

#274 added a required `impactedTestsEnabled` field to the SDK's SettingsResponse
model but didn't update the integration-test mock backend, so the settings
response it returned no longer decoded — failing every integration test that
boots the SDK against the mock.

Emit `impacted_tests_enabled` from MockBackend.buildSettingsResponse (configurable
via Settings.impactedTestsEnabled, default false). Matches the field set the SDK
decoder requires, as covered by LibraryConfigurationServiceThrowTests.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

* Add telemetry integration coverage; flush telemetry synchronously at shutdown

Verifying telemetry end-to-end surfaced a real bug: Telemetry.shutdown() drained
the in-memory store to the exporter and enqueued app-closing, but only called
exporter.shutdown() (stop the worker) — never exporter.flush(). Since flush() is
what synchronously uploads, the final batch (self-metrics + app-closing) sat on
disk unsent at process exit; only the direct app-started POST reliably reached
the backend. Add exporter.flush() before shutdown so the last batch is uploaded.

Tooling to assert telemetry reaches the backend:
- MockBackend.parseTelemetry decodes the captured /api/v2/apmtelemetry bodies
  (both the direct single-payload form and the message-batch envelope) into typed
  TelemetrySeries + observed event types, exposed via Requests.telemetry* accessors.
- MockBackendTelemetryParsingTests unit-tests the parser against the wire format.
- A telemetryReported integration test asserts app-started/app-closing events and
  generate-metrics (test_session) round-trip to the backend over a basic run.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>