void checkInConnection(String connectionId) {
    HostPortPoolManager hostPortPoolManager = connectionIdToPoolManager.get(connectionId);
    if (hostPortPoolManager == null) {
      throw new IllegalArgumentException("Invalid connection id passed in");
    }
    hostPortPoolManager.checkInConnection(connectionId);
  }
Add connection to available pool.
@param connectionId the id of the newly established or previously checked out connection.
@throws {@link IllegalArgumentException} if the passed in connection id is invalid.
ambry-network/src/main/java/com/github/ambry/network/ConnectionTracker.java