[optimze] optimize some metric style

[fansehep]

In /curvefs/src/client/metric/client_metric.h:

struct FSMetric {
    const std::string prefix = "curvefs_client";

...

struct S3Metric {
    const std::string prefix = "curvefs_s3";

...
struct DiskCacheMetric {
    const std::string prefix = "curvefs_disk_cache";

struct KVClientMetric {
    const std::string prefix = "curvefs_kvclient";

...

struct KVClientMetric {
    const std::string prefix = "curvefs_kvclient";

...

Use const std::string xxx in a struct type is strange. static is right.

Optimize them in all *.h and *.cpp;

[Veeshavanachu]

i will take this

[ashutosh887]

@fansehep Let me work on this

[caoxianfei1]

@ashutosh887 the work going on?

[wu-hanqing]

struct FSMetric {
    static constexpr const char* prefix = "curvefs_client";
};

maybe this's better, if you use static const std::string, you have to define the string in cpp file.

godbolt

[fansehep]

const char* if we have a function like void xxx(const std::string& str_name), we input a const char* here, that will construct a string.

But in fact, static variables are initialized at runtime, and char* can be compile-time constants. It may depend on the scenario.

I think here std::string is better than const char*.

[zhanghuidinah]

• This issue has been chosen for community developer activities. Welcome to feel free to try it. After the PR is merged, you will receive a gift package and a certificate as a community contributor.

• Developer Activities Registration Steps please ref [2023 Q2 Developer Activities]: Call For Participation！ #2334 (comment)

[Xinlong-Chen]

The issue seems more simple than other issues, can I get this task?
I agree with the const static std::string plan because it is more high-efficiency.
I will add a cpp file to initialize all the string variables.

[Xinlong-Chen]

Why does following classes not define const? (such as const std::string prefix;，other class in this file do it)
In /curvefs/src/client/metric/client_metric.h：

struct MDSClientMetric {
    std::string prefix;
}

struct MetaServerClientMetric {
    std::string prefix;
}

struct ClientOpMetric {
    std::string prefix;
}

[wu-hanqing]

it should be, you can fix this.

[czzmiku]

can I do it? #2319