HttpClient connection pool exhausted

Issue

Many threads in the following state, waiting to acquire a connection from the HttpClient connection pool:

"ForkJoinPool.commonPool-worker-0" #2484 daemon prio=5 os_prio=0 tid=0x00007fba9400c000 nid=0xad2 waiting on condition [0x00007fbb617d9000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x000000064bd3ae88> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
    at org.apache.http.pool.AbstractConnPool.getPoolEntryBlocking(AbstractConnPool.java:393)
    at org.apache.http.pool.AbstractConnPool.access$300(AbstractConnPool.java:70)
    at org.apache.http.pool.AbstractConnPool$2.get(AbstractConnPool.java:253)
    - locked <0x00000006e1700198> (a org.apache.http.pool.AbstractConnPool$2)
    at org.apache.http.pool.AbstractConnPool$2.get(AbstractConnPool.java:198)
    at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.leaseConnection(PoolingHttpClientConnectionManager.java:304)
    at org.apache.http.impl.conn.PoolingHttpClientConnectionManager$1.get(PoolingHttpClientConnectionManager.java:280)
    at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:190)
    at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:186)
    at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:89)
    at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:110)
    at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:185)
    at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:83)
    at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:56)
    at org.springframework.http.client.HttpComponentsClientHttpRequest.executeInternal(HttpComponentsClientHttpRequest.java:87)
    at org.springframework.http.client.AbstractBufferingClientHttpRequest.executeInternal(AbstractBufferingClientHttpRequest.java:48)
    at org.springframework.http.client.AbstractClientHttpRequest.execute(AbstractClientHttpRequest.java:53)
    at org.springframework.http.client.InterceptingClientHttpRequest$InterceptingRequestExecution.execute(InterceptingClientHttpRequest.java:109)
    at org.springframework.boot.actuate.metrics.web.client.MetricsClientHttpRequestInterceptor.intercept(MetricsClientHttpRequestInterceptor.java:93)
    at org.springframework.http.client.InterceptingClientHttpRequest$InterceptingRequestExecution.execute(InterceptingClientHttpRequest.java:93)
    at brave.spring.web.TracingClientHttpRequestInterceptor.intercept(TracingClientHttpRequestInterceptor.java:55)
    at org.springframework.cloud.sleuth.instrument.web.client.LazyTracingClientHttpRequestInterceptor.intercept(TraceWebClientAutoConfiguration.java:309)
    at org.springframework.http.client.InterceptingClientHttpRequest$InterceptingRequestExecution.execute(InterceptingClientHttpRequest.java:93)
    at org.springframework.http.client.InterceptingClientHttpRequest.executeInternal(InterceptingClientHttpRequest.java:77)
    at org.springframework.http.client.AbstractBufferingClientHttpRequest.executeInternal(AbstractBufferingClientHttpRequest.java:48)
    at org.springframework.http.client.AbstractClientHttpRequest.execute(AbstractClientHttpRequest.java:53)
    at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:742)
    at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:717)
    at org.springframework.web.client.RestTemplate.exchange(RestTemplate.java:605)
...