fix h2 rpc_dump memory leak bug

[GreateCode]

What problem does this PR solve?

Issue Number:

Problem Summary:
h2 rpc_dump时 内存泄漏。

具体是http和h2都走ProcessHttpRequest方法，在处理rpc_dump时，先AskToBeSampled创建堆对象再判is_http2，若是http2无释放堆对象逻辑。

发压cmd：rpc_press -protocol=h2 xxxx

What is changed and the side effects?

Changed:
h2 req不做AskToBeSampled

修复前：

brpc_h2_rpc_dump_mem_leak_bug.pdf

修复后：

brpc_h2_rpc_dump_mem_leak_fixed.pdf

Side effects:

• Performance effects(性能影响):
  无
• Breaking backward compatibility(向后兼容性):

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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).

[GreateCode]

@chenBright

[chenBright]

LGTM

[GreateCode]

@wwbmmm 合下？

[yanglimingcn]

LGTM