What is Spring Boot Actuator and what does it provide in Boot 4?

# application.yml: Actuator configuration in Boot 4
management:
  endpoints:
    web:
      exposure:
        include: health,info,metrics,loggers  # expose specific endpoints
        # include: "*"  # expose all (not recommended in production)
  endpoint:
    health:
      show-details: when-authorized
  opentelemetry:            # Boot 4: new OTel-specific config namespace
    resource-attributes:
      service.name: order-service
      service.version: "2.1.0"
  tracing:
    sampling:
      probability: 1.0  # 100% sampling in dev; reduce in prod

# Custom health indicator:
@Component
public class DatabaseHealthIndicator implements HealthIndicator {
    @Override
    public Health health() {
        try {
            jdbcTemplate.queryForObject("SELECT 1", Integer.class);
            return Health.up()
                .withDetail("database", "reachable")
                .build();
        } catch (Exception e) {
            return Health.down()
                .withDetail("error", e.getMessage())
                .build();
        }
    }
}