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, Prometheus metrics, JMX reporting, and Zipkin tracing based on their respective configuration flags, then registers health checks and wires up the Guava ServiceManager.
Parameters
servletInit
An optional block to register additional servlets in the ServletGroup.