initServletService
Initializes the servlet service, metrics, Zipkin tracing, and health checks.
This method should be called during subclass initialization. It conditionally sets up admin servlets via an embedded Jetty server, then delegates to initMetricsAndHealthChecks to wire up Prometheus metrics, JMX reporting, Zipkin tracing, health checks, and the Guava ServiceManager.
Parameters
servletInit
An optional block to register additional servlets in the ServletGroup.