fun queuedCalls(): List<Call> =
    this.withLock {
      return Collections.unmodifiableList(readyAsyncCalls.map { it.call })
    }

  /** Returns a snapshot of the calls currently being executed. */
  fun runningCalls(): List<Call> =
    this.withLock {
      return Collections.unmodifiableList(runningSyncCalls + runningAsyncCalls.map { it.call })
    }

  fun queuedCallsCount(): Int = this.withLock { readyAsyncCalls.size }