add gflag to disable sampler thread

[leaf-potato]

What problem does this PR solve?

Issue Number: #1891

Problem Summary: there is currently no gflag to control the switch of the sampler thread

What is changed and the side effects?

Changed: add --bvar_enable_sampling flag to control the switch of the sampler thread. if flag is false, does not create a global singleton SamplerCollector object.

Side effects:

• Performance effects(性能影响): setting the flag to false will reduce the creation of one thread and the usage of CPU and memory

• Breaking backward compatibility(向后兼容性): yes

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).

[wwbmmm]

LGTM

[serverglen]

LGTM