fix: Resolve core issues at runtime

[Mixficsol]

在 GetHistogram () 方法中的 histogram_family_->Add 这里会出现 core dump，因为 histogram_family_ 会每分钟重置一次，由于在重置的时候没有对 histogram_family_ 有上锁的操作，这里会产生在调用 Add 的情况下 histogram_family_ 是空指针导致进程崩溃

prometheus::Histogram& PikaCmdTableManager::GetHistogram(const std::string& opt) {
  {
    std::shared_lock<std::shared_mutex> read_lock(histograms_mutex_);
    auto it = histograms_.find(opt);
    if (it != histograms_.end()) {
      return *(it->second);
    }
  }
  std::unique_lock<std::shared_mutex> write_lock(histograms_mutex_);
  auto& new_histogram = histogram_family_->Add(  // 这里可能会产生崩溃
    {{"command", opt}}, 
    prometheus::Histogram::BucketBoundaries{0.5, 1, 2, 3, 5, 7, 10, 15, 20, 30, 40, 50, 65, 75, 85, 100, 125, 140, 150, 160, 175, 185, 200, 300, 400, 500, 750, 1000, 2000, 5000, 10000}
  );
  histograms_[opt] = &new_histogram; // 这里可能会产生崩溃
  return new_histogram;
}

prometheus::Family<prometheus::Histogram>* PikaCmdTableManager::GetHistograms() {
  std::shared_lock<std::shared_mutex> read_lock(histograms_mutex_);  // 增加了读锁保护 histogram_family_
  return histogram_family_;
}

在修复后，新增加了在重置 histogram_family_ 上了一个互斥锁进行保护

void PikaCmdTableManager::ResetCommandCount() {
  {
    std::unique_lock<std::shared_mutex> write_lock(slow_command_mutex_); 
    slow_command_count_.clear();
  }

  {
    std::unique_lock<std::shared_mutex> write_lock(histograms_mutex_); // 在重置的时候新加的互斥锁保护 histogram_family_ 是线程安全的  
    InitHistograms();                
  }
}

void PikaCmdTableManager::InitHistograms() {
  histograms_.clear();
  prometheus_registry_ = std::make_shared<prometheus::Registry>();
  histogram_family_ = &prometheus::BuildHistogram()
      .Name("pika_command_duration_seconds")
      .Help("Execution time of Pika commands in seconds")
      .Register(*prometheus_registry_);
}

PikaCmdTableManager::PikaCmdTableManager() {
  cmds_ = std::make_unique<CmdTable>();
  cmds_->reserve(300);
  InitHistograms();
}

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