Recheck for null service instance to avoid micrometer NPE. (#1197)

Author: OlgaMaciaszek

docs/src/main/asciidoc/_configprops.adoc

@@ -53,6 +53,7 @@
 |spring.cloud.loadbalancer.retry.retry-on-all-operations | `+++false+++` | Indicates retries should be attempted on operations other than `HttpMethod.GET`}.
 |spring.cloud.loadbalancer.retry.retryable-status-codes | `+++{}+++` | A `Set` of status codes that should trigger a retry.
 |spring.cloud.loadbalancer.service-discovery.timeout |  | String representation of Duration of the timeout for calls to service discovery.
+|spring.cloud.loadbalancer.stats.micrometer.enabled | `+++false+++` | Enables Spring Cloud LoadBalancer Micrometer stats.
 |spring.cloud.loadbalancer.sticky-session.add-service-instance-cookie | `+++false+++` | Indicates whether a cookie with the newly selected instance should be added by LoadBalancer.
 |spring.cloud.loadbalancer.sticky-session.instance-id-cookie-name | `+++sc-lb-instance-id+++` | The name of the cookie holding the preferred instance id.
 |spring.cloud.loadbalancer.use-raw-status-code-in-response-data | `+++false+++` | Indicates that raw status codes should be used in {@link ResponseData}.

spring-cloud-loadbalancer/src/main/java/org/springframework/cloud/loadbalancer/blocking/client/BlockingLoadBalancerClient.java

@@ -105,6 +105,9 @@ private <T> TimedRequestContext buildRequestContext(LoadBalancerRequest<T> deleg
 	@Override
 	public <T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request)
 			throws IOException {
+		if (serviceInstance == null) {
+			throw new IllegalArgumentException("Service Instance cannot be null");
+		}
 		DefaultResponse defaultResponse = new DefaultResponse(serviceInstance);
 		Set<LoadBalancerLifecycle> supportedLifecycleProcessors = getSupportedLifecycleProcessors(serviceId);
 		Request lbRequest = request instanceof Request ? (Request) request : new DefaultRequest<>();

spring-cloud-loadbalancer/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -33,6 +33,12 @@
 			"name": "spring.cloud.loadbalancer.enabled",
 			"description": "Enables Spring Cloud LoadBalancer.",
 			"type": "java.lang.Boolean"
+		},
+		{
+			"defaultValue": "false",
+			"name": "spring.cloud.loadbalancer.stats.micrometer.enabled",
+			"description": "Enables Spring Cloud LoadBalancer Micrometer stats.",
+			"type": "java.lang.Boolean"
 		}
 	]
 }