Update docs for skywalking tracer (#14210)

Commit Message: update docs for skywalking tracer
Additional Description:

Add some docs for SkyWalking tracer. This is a supplement for #13060. It has been delayed until now for personal reasons.

Risk Level: Low
Testing: N/A
Docs Changes: Docs Only
Release Notes: N/A
Platform Specific Features: N/A

Signed-off-by: wbpcode <comems@msn.com>

docs/root/configuration/http/http_conn_man/headers.rst

@@ -460,6 +460,16 @@ The integer value indicates the sampling decision that has been made for this tr
 A value of 0 indicates that the trace should not be collected, and a value of 1
 requests that spans are sampled and reported.
 
+.. _config_http_conn_man_headers_sw8:
+
+sw8
+----------
+
+The *sw8* HTTP header is used by the SkyWalking tracer in Envoy. It contains the key
+tracing context for the SkyWalking tracer and is used to establish the relationship between
+the tracing spans of downstream and Envoy. See more on SkyWalking tracing
+`here <https://github.com/apache/skywalking/blob/v8.1.0/docs/en/protocols/Skywalking-Cross-Process-Propagation-Headers-Protocol-v3.md>`__.
+
 .. _config_http_conn_man_headers_custom_request_headers:
 
 Custom request/response headers

docs/root/intro/arch_overview/observability/tracing.rst

@@ -19,8 +19,8 @@ sources of latency. Envoy supports three features related to system wide tracing
   providers, that are divided into two subgroups:
 
   - External tracers which are part of the Envoy code base, like `LightStep <https://lightstep.com/>`_,
-    `Zipkin <https://zipkin.io/>`_  or any Zipkin compatible backends (e.g. `Jaeger <https://github.com/jaegertracing/>`_), and
-    `Datadog <https://datadoghq.com>`_.
+    `Zipkin <https://zipkin.io/>`_  or any Zipkin compatible backends (e.g. `Jaeger <https://github.com/jaegertracing/>`_),
+    `Datadog <https://datadoghq.com>`_ and `SkyWalking <http://skywalking.apache.org/>`_.
   - External tracers which come as a third party plugin, like `Instana <https://www.instana.com/blog/monitoring-envoy-proxy-microservices/>`_.
 
 Support for other tracing providers would not be difficult to add.
@@ -82,6 +82,10 @@ Alternatively the trace context can be manually propagated by the service:
   :ref:`config_http_conn_man_headers_x-datadog-parent-id`,
   :ref:`config_http_conn_man_headers_x-datadog-sampling-priority`).
 
+* When using the SkyWalking tracer, Envoy relies on the service to propagate the
+  SkyWalking-specific HTTP headers (
+  :ref:`config_http_conn_man_headers_sw8`).
+
 What data each trace contains
 -----------------------------
 An end-to-end trace is comprised of one or more spans. A