Add support for B3Multi headers propagation

Some services expect [B3 Multi][1] headers as input information.
To support that we need to be able to Inject them into upstream requests. Lowercase headers used to be [compatible][2] with Istio Envoy.

Tests will be added as a separate commit later on.

Solving #36

[1]: https://github.com/openzipkin/b3-propagation#multiple-headers
[2]: open-telemetry/opentelemetry-go#765

Author: Vladimir Kuznichenkov

instrumentation/nginx/README.md

@@ -171,7 +171,7 @@ be started. The default propagator is W3C.
 The same inheritance rules as [`proxy_set_header`](http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_set_header) apply, which means this directive is applied at the current configuration level if and only if there are no `proxy_set_header` directives defined on a lower level.
 
 - **required**: `false`
-- **syntax**: `opentelemetry_propagate` or `opentelemetry_propagate b3`
+- **syntax**: `opentelemetry_propagate` or `opentelemetry_propagate b3` or `opentelemetry_propagate b3multi`
 - **block**: `http`, `server`, `location`
 
 ### `opentelemetry_capture_headers`
@@ -232,6 +232,7 @@ The following nginx variables are set by the instrumentation:
   context](https://www.w3.org/TR/trace-context/#trace-context-http-headers-format), e.g.: `00-0af7651916cd43dd8448eb211c80319c-b9c7c989f97918e1-01`
 - `opentelemetry_context_b3` - Trace context in the [B3
   format](https://github.com/openzipkin/b3-propagation#single-header). Only set when using `opentelemetry_propagate b3`.
+- `opentelemetry_sampled` - does current Span records information, "1" or "0"
 - `opentelemetry_trace_id` - Trace Id of the current span
 - `opentelemetry_span_id` - Span Id of the current span
 

instrumentation/nginx/src/otel_ngx_module.cpp

@@ -136,6 +136,9 @@ OtelGetTraceId(ngx_http_request_t* req, ngx_http_variable_value_t* v, uintptr_t
 static ngx_int_t
 OtelGetSpanId(ngx_http_request_t* req, ngx_http_variable_value_t* v, uintptr_t data);
 
+static ngx_int_t
+OtelGetSampled(ngx_http_request_t* req, ngx_http_variable_value_t* v, uintptr_t data);
+
 static ngx_http_variable_t otel_ngx_variables[] = {
   {
     ngx_string("otel_ctx"),
@@ -169,6 +172,14 @@ static ngx_http_variable_t otel_ngx_variables[] = {
     NGX_HTTP_VAR_NOCACHEABLE | NGX_HTTP_VAR_NOHASH,
     0,
   },
+  {
+    ngx_string("opentelemetry_sampled"),
+    nullptr,
+    OtelGetSampled,
+    0,
+    NGX_HTTP_VAR_NOCACHEABLE | NGX_HTTP_VAR_NOHASH,
+    0,
+  },
   ngx_http_null_variable,
 };
 
@@ -217,6 +228,38 @@ nostd::string_view WithoutOtelVarPrefix(ngx_str_t value) {
   return {(const char*)value.data + prefixLength, value.len - prefixLength};
 }
 
+static ngx_int_t
+OtelGetSampled(ngx_http_request_t* req, ngx_http_variable_value_t* v, uintptr_t data) {
+  TraceContext* traceContext = GetTraceContext(req);
+
+  if (traceContext == nullptr || !traceContext->request_span) {
+    ngx_log_error(
+        NGX_LOG_ERR, req->connection->log, 0,
+        "Unable to get trace context when getting span id");
+    return NGX_OK;
+  }
+
+  trace::SpanContext spanContext = traceContext->request_span->GetContext();
+
+  if (spanContext.IsValid()) {
+    u_char* isSampled = spanContext.trace_flags().IsSampled() ? (u_char*) "1" : (u_char*) "0";
+
+    v->len = strlen((const char*)isSampled);
+    v->valid = 1;
+    v->no_cacheable = 1;
+    v->not_found = 0;
+    v->data = isSampled;
+  } else {
+    v->len = 0;
+    v->valid = 0;
+    v->no_cacheable = 1;
+    v->not_found = 1;
+    v->data = nullptr;
+  }
+
+  return NGX_OK;
+}
+
 static ngx_int_t
 OtelGetTraceContextVar(ngx_http_request_t* req, ngx_http_variable_value_t* v, uintptr_t data) {
   if (!IsOtelEnabled(req)) {
@@ -758,6 +801,17 @@ std::vector<HeaderPropagation> B3PropagationVars() {
   };
 }
 
+std::vector<HeaderPropagation> B3MultiPropagationVars() {
+  return {
+    {"proxy_set_header", "x-b3-traceid", "$opentelemetry_trace_id"},
+    {"proxy_set_header", "x-b3-spanid", "$opentelemetry_span_id"},
+    {"proxy_set_header", "x-b3-sampled", "$opentelemetry_sampled"},
+    {"fastcgi_param", "HTTP_B3_TRACEID", "$opentelemetry_trace_id"},
+    {"fastcgi_param", "HTTP_B3_SPANID", "$opentelemetry_span_id"},
+    {"fastcgi_param", "HTTP_B3_SAMPLED", "$opentelemetry_sampled"},
+  };
+}
+
 std::vector<HeaderPropagation> OtelPropagationVars() {
   return {
     {"proxy_set_header", "traceparent", "$opentelemetry_context_traceparent"},
@@ -778,6 +832,8 @@ char* OtelNgxSetPropagation(ngx_conf_t* conf, ngx_command_t*, void* locConf) {
 
     if (propagationType == "b3") {
       locationConf->propagationType = TracePropagationB3;
+    } else if (propagationType == "b3multi") {
+      locationConf->propagationType = TracePropagationB3Multi;
     } else if (propagationType == "w3c") {
       locationConf->propagationType = TracePropagationW3C;
     } else {
@@ -791,6 +847,8 @@ char* OtelNgxSetPropagation(ngx_conf_t* conf, ngx_command_t*, void* locConf) {
   std::vector<HeaderPropagation> propagationVars;
   if (locationConf->propagationType == TracePropagationB3) {
     propagationVars = B3PropagationVars();
+  } else if (locationConf->propagationType == TracePropagationB3Multi) {
+    propagationVars = B3MultiPropagationVars();
   } else {
     propagationVars = OtelPropagationVars();
   }

instrumentation/nginx/src/propagate.cpp

@@ -79,6 +79,9 @@ opentelemetry::context::Context ExtractContext(OtelCarrier* carrier) {
     case TracePropagationW3C: {
       return OtelW3CPropagator().Extract(textMapCarrier, root);
     }
+    case TracePropagationB3Multi: {
+      return OtelB3MultiPropagator().Extract(textMapCarrier, root);
+    }
     case TracePropagationB3: {
       if (HasHeader(carrier->req, "b3")) {
         return OtelB3Propagator().Extract(textMapCarrier, root);
@@ -104,6 +107,10 @@ void InjectContext(OtelCarrier* carrier, opentelemetry::context::Context context
       OtelB3Propagator().Inject(textMapCarrier, context);
       break;
     }
+    case TracePropagationB3Multi: {
+      OtelB3MultiPropagator().Inject(textMapCarrier, context);
+      break;
+    }
     default:
       break;
   }

instrumentation/nginx/src/trace_context.h

@@ -17,6 +17,7 @@ enum TracePropagationType {
   TracePropagationUnset,
   TracePropagationW3C,
   TracePropagationB3,
+  TracePropagationB3Multi,
 };
 
 struct TraceContext {