Bugfix: bthread_worker_usage would be greater than bthread_worker_count

[chenBright]

What problem does this PR solve?

Issue Number: resolve #1231 , resolve #2640 , resolve #2972

Problem Summary:

What is changed and the side effects?

Changed:

bthread_worker_usage should consist of _cumulated_cputime_ns and the CPU time of the task currently run by the worker.

Side effects:

• Performance effects:

• Breaking backward compatibility:

---

Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[chenBright]

@wwbmmm @yanglimingcn Please take a look.

[chenBright]

@wwbmmm @yanglimingcn Friendly ping!

[chenBright]

void* test(void*) {
    while (!brpc::IsAskedToQuit()) {
        sleep(2);
        bthread_yield();
    }
}

for (int i = 0; i < 3; ++i) {
    bthread_t tid;
    bthread_start_background(&tid, NULL, test, NULL);
}

Start 3 bthreads and loop sleep for 2s. The value of bthread_worker_usage is correct, and the extra 1 comes from the epoll bthread.

[wwbmmm]

LGTM