Set http default method to "/"

[chenBright]

What problem does this PR solve?

Issue Number:

Problem Summary:

What is changed and the side effects?

Changed:

Side effects:

• Performance effects(性能影响):

• 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).

[wwbmmm]

这个是否会影响代码的向前兼容性？

[chenBright]

应该不影响，目前method name只在创建client span的时候使用了而已。

brpc/src/brpc/channel.cpp

Lines 470 to 479 in fc885ad

	if (_get_method_name) {
	method_name = &_get_method_name(method, cntl);
	} else if (method) {
	method_name = &method->full_name();
	} else {
	const static std::string NULL_METHOD_STR = "null-method";
	method_name = &NULL_METHOD_STR;
	}
	Span* span = Span::CreateClientSpan(
	*method_name, start_send_real_us - start_send_us);

[chenBright]

如果担心有影响的话，也可以在CallMethod中，_get_method_name后method_name为空再设置为"/"。

[wwbmmm]

LGTM