feat: extract inbound W3C trace context in HTTP server

When an inbound request carries traceparent/tracestate headers,
extract them via propagation.extract() and scope the entire
request processing under that context. This connects cloudserver
spans to upstream callers (e.g. NGINX, Beyla) as a single
distributed trace.

Issue: CLDSRV-884

Author: delthas

lib/server.js

@@ -7,6 +7,9 @@ const { setServerHeader } = arsenal.s3routes.routesUtils;
 const { RedisClient, StatsClient } = arsenal.metrics;
 const monitoringClient = require('./utilities/monitoringHandler');
 
+const enableOtel = process.env.ENABLE_OTEL === 'true';
+const otelApi = enableOtel ? require('@opentelemetry/api') : null;
+
 const logger = require('./utilities/logger');
 const { internalHandlers } = require('./utilities/internalHandlers');
 const { clientCheck, healthcheckHandler } = require('./utilities/healthcheckHandler');
@@ -206,7 +209,23 @@ class S3Server {
                 vault,
             },
         };
-        arsenal.s3routes.routes(req, res, params, logger, this.config);
+
+        // Extract inbound W3C trace context (traceparent/tracestate) and
+        // run the entire request within it so that spans created downstream
+        // are children of the caller's trace.
+        if (enableOtel && req.headers.traceparent) {
+            const traceHeaders = { traceparent: req.headers.traceparent };
+            if (req.headers.tracestate) {
+                traceHeaders.tracestate = req.headers.tracestate;
+            }
+            const remoteContext = otelApi.propagation.extract(
+                otelApi.context.active(), traceHeaders);
+            return otelApi.context.with(remoteContext, () => {
+                arsenal.s3routes.routes(req, res, params, logger, this.config);
+            });
+        }
+
+        return arsenal.s3routes.routes(req, res, params, logger, this.config);
     }
 
     /**