Annotate interfaces with @FunctionaInterface

Closes gh-6857

Author: eddumelendez

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/cache/CacheStatisticsProvider.java

@@ -27,6 +27,7 @@
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface CacheStatisticsProvider<C extends Cache> {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/PublicMetrics.java

@@ -29,6 +29,7 @@
  * @author Dave Syer
  * @see SystemPublicMetrics SystemPublicMetrics for an example implementation
  */
+@FunctionalInterface
 public interface PublicMetrics {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/EndpointHandlerMappingCustomizer.java

@@ -22,6 +22,7 @@
  * @author Dave Syer
  * @since 1.2.0
  */
+@FunctionalInterface
 public interface EndpointHandlerMappingCustomizer {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/ManagementServletContext.java

@@ -22,6 +22,7 @@
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface ManagementServletContext {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthAggregator.java

@@ -34,6 +34,7 @@
  * @author Christian Dupuis
  * @since 1.1.0
  */
+@FunctionalInterface
 public interface HealthAggregator {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthIndicator.java

@@ -22,6 +22,7 @@
  * @author Dave Syer
  * @see ApplicationHealthIndicator
  */
+@FunctionalInterface
 public interface HealthIndicator {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/info/InfoContributor.java

@@ -22,6 +22,7 @@
  * @author Stephane Nicoll
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface InfoContributor {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/GaugeService.java

@@ -27,6 +27,7 @@
  *
  * @author Dave Syer
  */
+@FunctionalInterface
 public interface GaugeService {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/export/Exporter.java

@@ -27,6 +27,7 @@
  * @author Dave Syer
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface Exporter {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/opentsdb/OpenTsdbNamingStrategy.java

@@ -22,6 +22,7 @@
  * @author Dave Syer
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface OpenTsdbNamingStrategy {
 
 	/**