Add WARN log and docs for Client bean Singleton requirement

- Added WARN log when Client bean for same contextId is resolved
  more than once to alert developers of potential misconfiguration
- Added documentation warning that Client bean must be Singleton
  scoped to avoid connection pool exhaustion

Closes #1308

Signed-off-by: gokid96 <gokid360@gmail.com>

Author: gokid96

docs/modules/ROOT/pages/spring-cloud-openfeign.adoc

@@ -138,6 +138,12 @@ Spring Cloud OpenFeign provides the following beans by default for feign (`BeanT
 * `Client` feignClient: If Spring Cloud LoadBalancer is on the classpath, `FeignBlockingLoadBalancerClient` is used.
 If none of them is on the classpath, the default Feign client is used.
 
+WARNING: The `Client` bean must be configured as a Singleton.
+If the `Client` bean is not a Singleton, a new connection pool may be created on each
+`FeignClient` initialization, which can lead to connection pool exhaustion and resource leaks.
+If you see a warning log about a `FeignClient` with a given `contextId` being initialized
+more than once, verify that your custom `Client` bean is Singleton scoped.
+
 NOTE: `spring-cloud-starter-openfeign` supports `spring-cloud-starter-loadbalancer`. However, as is an optional dependency, you need to make sure it has been added to your project if you want to use it.
 
 When it comes to the Apache HttpClient 5-backed Feign clients, it's enough to ensure HttpClient 5 is on the classpath, but you can still disable its use for Feign Clients by setting  `spring.cloud.openfeign.httpclient.hc5.enabled` to `false`.

spring-cloud-openfeign-core/src/main/java/org/springframework/cloud/openfeign/FeignClientFactoryBean.java

@@ -18,10 +18,13 @@
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
 
 import feign.Capability;
@@ -85,6 +88,8 @@ public class FeignClientFactoryBean
 
 	private static final Log LOG = LogFactory.getLog(FeignClientFactoryBean.class);
 
+	private static final Set<String> resolvedContextIds = Collections.newSetFromMap(new ConcurrentHashMap<>());
+
 	private Class<?> type;
 
 	private String name;
@@ -488,6 +493,14 @@ <T> T getTarget() {
 		// (e.g., connection pools, threads) if not properly managed.
 		Client client = getOptional(feignClientFactory, Client.class);
 		if (client != null) {
+			if (!resolvedContextIds.add(contextId)) {
+				if (LOG.isWarnEnabled()) {
+					LOG.warn("FeignClient with contextId '" + contextId
+						+ "' is being initialized more than once. "
+						+ "Ensure the Client bean is Singleton scoped "
+						+ "to avoid connection pool exhaustion.");
+				}
+			}
 			if (client instanceof FeignBlockingLoadBalancerClient) {
 				// not load balancing because we have a url,
 				// but Spring Cloud LoadBalancer is on the classpath, so unwrap