Implement x-failure-percentage header for fault injection control (#122)

## ✅ Implemented `x-failure-percentage` header for fault injection
control

This PR addresses issue #80 by implementing header-based fault
injection, providing a better alternative to `MeshFaultInjection` for
creating realistic and targeted demos.

Fixes #80

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: lahabana <[email protected]>

Author: Copilot

README.md

@@ -114,5 +114,20 @@ To enforce response status code you need to set header `x-set-response-status-co
 curl localhost:5050/api/counter -XPOST -H "x-set-response-status-code: 503"
 ```
 
+### Simulating failures with fault injection
+
+To simulate random failures you need to set header `x-failure-percentage` with a value between 0 and 100. A value of 0 (or missing header) means no failures, while 100 means always fail. Example:
+
+```shell
+# 50% chance of failure
+curl localhost:5050/api/counter -XPOST -H "x-failure-percentage: 50"
+
+# Always fail
+curl localhost:5050/api/version -H "x-failure-percentage: 100"
+
+# No failures (same as not setting the header)
+curl localhost:5050/api/version -H "x-failure-percentage: 0"
+```
+
 [kuma-url]: https://kuma.io/
 [kuma-logo]: https://kuma-public-assets.s3.amazonaws.com/kuma-logo-v2.png

app/internal/base/handler.go

@@ -6,6 +6,7 @@ import (
 	"github.com/kumahq/kuma-counter-demo/pkg/api"
 	"go.opentelemetry.io/otel/trace"
 	"log/slog"
+	"math/rand"
 	"net/http"
 	"strconv"
 	"sync"
@@ -40,12 +41,23 @@ func (s *ServerImpl) writeResponse(w http.ResponseWriter, r *http.Request, origi
 	if originalStatusCode/100 < 5 { // If the original status is 5xx ignore our override
 		statusStr := r.Header.Get("x-set-response-status-code")
 		if st, _ := strconv.Atoi(statusStr); st != 0 {
-			originalStatusCode = st
+			statusCode = st
+		}
+		// Handle fault injection via x-failure-percentage header
+		if statusCode/100 < 4 { // Only inject faults for successful responses (2xx, 3xx)
+			failurePercentageStr := r.Header.Get("x-failure-percentage")
+			if failurePercentage, parseErr := strconv.Atoi(failurePercentageStr); parseErr == nil && failurePercentage > 0 {
+				if failurePercentage >= 100 || rand.Intn(100) < failurePercentage {
+					statusCode = http.StatusInternalServerError
+					err = fmt.Errorf("simulated failure with %d%% chance", failurePercentage)
+				}
+			}
 		}
 	}
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 	w.Header().Set("X-Demo-App-Version", s.version)
-	w.WriteHeader(originalStatusCode)
+	w.Header().Set("X-Original-Status-Code", strconv.Itoa(originalStatusCode))
+	w.WriteHeader(statusCode)
 	if err != nil {
 		slog.ErrorContext(r.Context(), "failed request with error", "err", err, "status", statusCode, "response", response)
 	} else {