fix: disable Prometheus metrics endpoint by default

BREAKING CHANGE: Prometheus metrics endpoint is now disabled by default
and must be explicitly enabled.

Changes:
- Set prometheus_enabled to false in config.exs (was true)
- Prevents port 9568 conflict errors when running multiple instances
- Must explicitly enable in config to expose metrics endpoint

This prevents the 'eaddrinuse' error when port 9568 is already in use
and allows Concord to be used as an embedded library without the
Prometheus overhead.

To enable Prometheus metrics:
- Set prometheus_enabled: true in your config
- Or set CONCORD_PROMETHEUS_ENABLED=true environment variable

Author: GSMLG-BOT

config/config.exs

@@ -50,8 +50,8 @@ config :concord,
     # TLS versions to support (default: TLS 1.2 and 1.3)
     versions: [:"tlsv1.2", :"tlsv1.3"]
   ],
-  # Prometheus metrics configuration
-  prometheus_enabled: true,
+  # Prometheus metrics configuration (disabled by default)
+  prometheus_enabled: false,
   prometheus_port: 9568,
   # OpenTelemetry distributed tracing configuration
   # Enable distributed tracing