Add OpenTelemetry integration

- Add OpenTelemetry dependencies to requirements.txt
- Implement OpenTelemetry configuration with feature flag support
- Add OpenTelemetry middleware for custom metrics and tracing
- Update Django settings to conditionally enable OpenTelemetry
- Add comprehensive test suite for OpenTelemetry functionality
- Create test scripts for running with/without OpenTelemetry
- Add pytest markers for OpenTelemetry test categorization
- Update documentation with OpenTelemetry setup and infrastructure details

Features:
- Feature flag controlled (ENABLE_OPENTELEMETRY) for F/LOSS deployments
- Localhost-only security model
- Custom avatar metrics and tracing
- Graceful fallback when OpenTelemetry is disabled
- Comprehensive test coverage for both enabled/disabled states

config.py

@@ -34,6 +34,9 @@ MIDDLEWARE.extend(
         "ivatar.middleware.CustomLocaleMiddleware",
     ]
 )
+
+# Add OpenTelemetry middleware only if feature flag is enabled
+# Note: This will be checked at runtime, not at import time
 MIDDLEWARE.insert(
     0,
     "ivatar.middleware.MultipleProxyMiddleware",
@@ -309,6 +312,13 @@ ENABLE_MALICIOUS_CONTENT_SCAN = True
 # Logging configuration - can be overridden in local config
 # Example: LOGS_DIR = "/var/log/ivatar"  # For production deployments
 
+# OpenTelemetry feature flag - can be disabled for F/LOSS deployments
+ENABLE_OPENTELEMETRY = os.environ.get("ENABLE_OPENTELEMETRY", "false").lower() in (
+    "true",
+    "1",
+    "yes",
+)
+
 # This MUST BE THE LAST!
 if os.path.isfile(os.path.join(BASE_DIR, "config_local.py")):
     from config_local import *  # noqa # flake8: noqa # NOQA # pragma: no cover