update OLTP doc (#555)

* update OLTP doc

* docs(swift): clarify OTLP gateway route is the configured endpoint

Codex review flagged that OTLPExporter never appends /v1/traces to
the endpoint — it POSTs to exactly the URL passed in. Describing the
gateway contract as "POST <your-url>/v1/traces" could lead readers to
double-append the suffix on their gateway route.

Co-authored-by: Corneliu Croitoru <cornelcroi@users.noreply.github.com>

---------

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Corneliu Croitoru <cornelcroi@users.noreply.github.com>

Author: cornelcroi

docs/src/content/docs/swift/tracing/built-in/otlp-exporter.md

@@ -123,6 +123,27 @@ let tracer = ProcessingTracer(
 )
 ```
 
+## Routing through your own gateway
+
+If you can't reach a backend directly from the device — or you don't want to ship its API key in the app — point `OTLPExporter` at an endpoint you control and let that server attach the real credentials and forward the traces on. The device holds only your own token; the backend key stays server-side.
+
+```swift
+let tracer = ProcessingTracer(
+    exporter: OTLPExporter(
+        endpoint: URL(string: "https://telemetry.example.com/v1/traces")!, // your gateway
+        headers: ["Authorization": "Bearer <app-token>"],                   // your auth, not the backend's
+        serviceName: "my-app"
+    )
+)
+```
+
+`OTLPExporter` doesn't care whether the URL is a backend or your own proxy — it POSTs the same OTLP/HTTP JSON either way, so no custom `TraceExporter` is needed. What your endpoint receives and how it should behave:
+
+- **`POST` to the configured `endpoint`** (in the example above, `/v1/traces`), `Content-Type: application/json`, body is an OTLP `ExportTraceServiceRequest`. It must return **2xx** or the batch is dropped (there is no retry — see [Error handling](#error-handling)).
+- **When it fires:** background, fire-and-forget — a batch is POSTed when `batchSize` spans accumulate or on `flush()`. Spans ship **as each one ends**, so a trace arrives across multiple POSTs and out of causal order (children before their parent root). Stitch by `traceId` + `parentSpanId` and tolerate late/orphan spans.
+- **What it can do:** at minimum, forward the body verbatim to the real backend with the backend's auth header added. It may also redact `gen_ai.prompt`/`gen_ai.completion`, route to different projects per user/env, buffer and retry, or fan out to another OTLP backend. An off-the-shelf [OpenTelemetry Collector](https://opentelemetry.io/docs/collector/) does the forward-and-inject case with no custom code.
+- **Payload notes (standard OTLP/JSON):** `intValue` fields are strings (`"1200"`), timestamps are string nanoseconds, `parentSpanId` is omitted (not null) on roots, and `status.code` is `0` (unset/ok) or `2` (error).
+
 ## Related pages
 
 - [Tracing Overview](/agent-squad/swift/tracing/overview/) — the full pipeline and all protocols.