Document proxy configuration for Java SDK (elastic#2100)

This PR adds a troubleshooting guide to help users resolve proxy issues
when using EDOT Java SDK. The guide explains that the Java SDK requires
explicit JVM proxy config.

Author: alexandra5000

troubleshoot/ingest/opentelemetry/edot-sdks/java/proxy-issues.md

@@ -0,0 +1,58 @@
+---
+navigation_title: Proxy issues
+description: Troubleshooting guide for the EDOT Java SDK, covering proxy issues.
+applies_to:
+  stack:
+  serverless:
+    observability:
+  product:
+    edot_java: ga
+products:
+  - id: cloud-serverless
+  - id: observability
+  - id: edot-sdk
+---
+
+# Proxy issues with EDOT Java SDK
+
+If your Java SDK sends telemetry but fails to communicate with the APM server, the issue might be due to missing or misconfigured proxy settings, which are required for outbound HTTP/S communication in some environments.
+
+## Symptoms
+
+These symptoms typically indicate that your Java SDK is experiencing a proxy issue:
+
+* Telemetry data is not received in Elastic APM after instrumenting the app.
+* Logs show errors like `Connection timed out`, `TLS handshake failure`, or other network-related issues when connecting to Elastic APM.
+
+## Resolution
+
+Set standard Java system properties to configure proxy settings. 
+
+Configure the following JVM system properties to enable outbound HTTP/HTTPS communication through a proxy:
+
+```bash
+java \
+-Dhttp.proxyHost=proxy.example.com \
+-Dhttp.proxyPort=8080 \
+-Dhttps.proxyHost=proxy.example.com \
+-Dhttps.proxyPort=8080 \
+```
+
+## Best practices
+
+You can test end-to-end connectivity to validate proxy routing. To confirm that your application or collector can reach the endpoint through the proxy, use `curl` with the `--proxy` option:
+
+```bash
+curl --proxy http://proxy.example.com:8080 https://example.elastic.co
+```
+
+If the proxy is working, you'll get a valid response. If not, common errors include:
+
+* `Connection timed out`
+* `Could not resolve host`
+* `Proxy CONNECT aborted`
+
+## Resources
+
+* [Java networking and proxies](https://docs.oracle.com/javase/8/docs/technotes/guides/net/proxies.html)
+* [Java agent configuration - upstream documentation](https://opentelemetry.io/docs/zero-code/java/agent/configuration/)

troubleshoot/ingest/opentelemetry/toc.yml

@@ -10,6 +10,8 @@ toc:
     children:
       - file: edot-sdks/dotnet/index.md
       - file: edot-sdks/java/index.md
+        children:
+          - file: edot-sdks/java/proxy-issues.md
       - file: edot-sdks/nodejs/index.md
       - file: edot-sdks/php/index.md
       - file: edot-sdks/python/index.md