Add documentation for tracing directive

src/docs/markdown/caddyfile/directives.md

@@ -62,6 +62,7 @@ Directive | Description
 **[route](/docs/caddyfile/directives/route)** | A group of directives treated literally as single unit
 **[templates](/docs/caddyfile/directives/templates)** | Execute templates on the response
 **[tls](/docs/caddyfile/directives/tls)** | Customize TLS settings
+**[tracing](/docs/caddyfile/directives/tracing)** | Integration with OpenTelemetry tracing
 **[try_files](/docs/caddyfile/directives/try_files)** | Rewrite that depends on file existence
 **[uri](/docs/caddyfile/directives/uri)** | Manipulate the URI
 
@@ -104,6 +105,8 @@ Because matcher tokens all work the same, the various possibilities for the matc
 Many directives manipulate the HTTP handler chain. The order in which those directives are evaluated matters, so a default ordering is hard-coded into Caddy:
 
 ```caddy-d
+tracing
+
 map
 root
 

src/docs/markdown/caddyfile/directives/tracing.md

@@ -0,0 +1,53 @@
+---
+title: tracing (Caddyfile directive)
+---
+
+# tracing
+
+It provides integration with OpenTelemetry tracing facilities.
+
+When enabled, it will propagate an existing trace context or initialize a new one.
+
+It is based on [github.com/open-telemetry/opentelemetry-go](https://github.com/open-telemetry/opentelemetry-go).
+
+It uses [gRPC](https://github.com/grpc/) as an exporter protocol and  W3C [tracecontext](https://www.w3.org/TR/trace-context/) and [baggage](https://www.w3.org/TR/baggage/) as propagators.
+
+## Syntax
+
+```caddy-d
+tracing {
+	[span <span_name>]
+}
+```
+
+- **&lt;span_name&gt;** - is a span name. Please see span naming [guidelines](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.7.0/specification/trace/api.md).
+
+## Configuration
+
+### Environment variables
+
+It can be configured using the environment variables defined
+by the [OpenTelemetry Environment Variable Specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md).
+
+For the exporter configuration details, please
+see [spec](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.7.0/specification/protocol/exporter.md).
+
+For example:
+
+```bash
+export OTEL_EXPORTER_OTLP_HEADERS="myAuthHeader=myToken,anotherHeader=value"
+export OTEL_EXPORTER_OTLP_TRACES_ENDPOINT=https://my-otlp-endpoint:55680
+```
+
+## Examples
+
+Here is a **Caddyfile** example:
+
+```
+handle /myHandler {
+	tracing {
+		span my-span
+	}
+	reverse_proxy 127.0.0.1:8081
+}
+```