Deprecated PushRegistry's numThreads property

This property is a left-over and was never used in Micrometer so this
commit deprecates its use so that it can be removed in the next feature
release.

Closes gh-20835

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/export/properties/PushRegistryProperties.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -92,10 +92,12 @@ public abstract class PushRegistryProperties {
 		this.readTimeout = readTimeout;
 	}
 
+	@Deprecated
 	public Integer getNumThreads() {
 		return this.numThreads;
 	}
 
+	@Deprecated
 	public void setNumThreads(Integer numThreads) {
 		this.numThreads = numThreads;
 	}

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/export/properties/PushRegistryPropertiesConfigAdapter.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -57,6 +57,7 @@ public abstract class PushRegistryPropertiesConfigAdapter<T extends PushRegistry
 	}
 
 	@Override
+	@SuppressWarnings("deprecation")
 	public int numThreads() {
 		return get(T::getNumThreads, PushRegistryConfig.super::numThreads);
 	}