initServletService
Initializes the servlet service, metrics, Zipkin tracing, and health checks.
This method should be called once during subclass initialization, after adding any extra services with addService. 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.
Throws
if the service has already been initialized.