docs(sdk): Add docs for w3c traceparent support (#14622)

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Add docs around w3c traceparent support.

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---------

Co-authored-by: Alex Krawiec <[email protected]>

Author: markushi

develop-docs/sdk/telemetry/traces/distributed-tracing/index.mdx

@@ -6,7 +6,7 @@ This document describes how an SDK should propagate information between differen
 
 For an overview see [Distributed Tracing](https://docs.sentry.io/product/performance/distributed-tracing/) in the product docs.
 
-Sentry uses two containers to hold trace information [`sentry-trace`](/sdk/performance/#header-sentry-trace) and [`baggage`](/sdk/performance/dynamic-sampling-context/#baggage-header).
+Sentry uses three containers to hold trace information [`sentry-trace`](/sdk/performance/#header-sentry-trace), [`baggage`](/sdk/performance/dynamic-sampling-context/#baggage-header) and optionally [`traceparent`](/sdk/performance/distributed-tracing/#w3c-trace-context-header).
 
 With these containers you can propagate a trace to a down-stream service by either:
 - adding `sentry-trace` and `baggage` HTTP headers (when making outgoing HTTP requests),
@@ -19,3 +19,11 @@ The SDK running in the receiving service needs to make sure to pick up incoming
 - reading the environment variables `SENTRY_TRACE` and `SENTRY_BAGGAGE` on startup.
 
 This trace information should be stored in the "propagation context" of the current scope. This makes sure that all telemetry that is emitted from the receiving service to Sentry will include the correct trace information.
+
+## W3C Trace Context Header
+
+To interoperate with OpenTelemetry (OTel) services, SDKs can optionally send the W3C Trace Context `traceparent` HTTP header on outgoing requests in addition to Sentry’s own `sentry-trace` and `baggage`. This enables traces originating from a Sentry SDK to be continued by components that only use OTel/W3C propagation.
+
+The `traceparent` header must only be sent when [`propagateTraceparent`](/sdk/telemetry/traces/#propagatetraceparent) is `true` and the request matches `tracePropagationTargets` (same condition as for `sentry-trace`/`baggage`).
+
+The full spec is available in the [W3C Trace Context](https://www.w3.org/TR/trace-context/) document.

develop-docs/sdk/telemetry/traces/index.mdx

@@ -115,6 +115,19 @@ The `maxSpans` should be implemented as an internal, non-configurable, constant
 
 The `maxSpans` limit may also help avoiding transactions that never finish (in platforms that keep a transaction open for as long as spans are open), preventing OOM errors, and generally avoiding degraded application performance.
 
+
+### `propagateTraceparent`
+
+This must be a boolean value. The default is `false`. This option is used to enable the propagation of the W3C Trace Context HTTP header `traceparent` on outgoing HTTP requests. This is useful when the receiving services only support OTel/W3C propagation.
+
+The `traceparent` header is only sent when `propagateTraceparent` is `true` and the request matches `tracePropagationTargets` (same condition as for `sentry-trace`/`baggage`).
+
+Header Format:
+- Key: `traceparent`
+- Value: `00-<traceId>-<spanId>-<sampled>` where `<sampled>` is `01` if sampled, otherwise `00`.
+
+The full spec is available in the [W3C Trace Context](https://www.w3.org/TR/trace-context/) specification.
+
 ## `Event` Changes
 
 As of writing, transactions are implemented as an extension of the `Event`