Can specify target address for rpc request

[wenxuwan]

What would you like to be added:

用户请求rpc的时候可以指定provider的地址，layotto直接将请求发到目标ip地址去，而不需要转发给mosn。用作测试使用。

Why is this needed:

rpc请求的时候，一个service可能会有多个provider，在调试的时候，期望的是可以发到目标provider上，而不是默认的走负载均衡策略。当前layotto会转发到mosn走mosn的服务发现策略，用户无法在不更改配置文件的前提下将请求发送到目标provider。

Design for this issue:

当前rpc请求的结构定义如下：

因此可以允许用户在grpc的header里面塞一个固定的字段rpc_target_address来指定目标provider的ip地址。

同时在channel的定义中增加directpool，用来区分和mosn pool的区别。

于此同时该pool大小设置为0，不做连接池、即用完关闭。

[kevinten10]

Hi，此功能是否可以兼容以下场景呢：

例如部署layotto + envoy，流量先经过layotto做处理，layotto再将流量直接转发给envoy。

• 流量不经过mosn。可能是 `app -> (http/grpc) -> layotto -> (http) -> envoy -> target service
• layotto->envoy走L7的http转发，而不是L4的ip转发。那是不是要加一个rpc_target_url

[wenxuwan]

Hi，此功能是否可以兼容以下场景呢：

例如部署layotto + envoy，流量先经过layotto做处理，layotto再将流量直接转发给envoy。

• 流量不经过mosn。可能是 `app -> (http/grpc) -> layotto -> (http) -> envoy -> target service
• layotto->envoy走L7的http转发，而不是L4的ip转发。那是不是要加一个rpc_target_url

当前的layotto是可以通过下面的配置将invoke转成http协议的。所以说理论上协议应该使用当前的protocol字段来配置，只用一个rpc_target_address字段来指定目标地址就可以了。

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity in the last 30 days. It will be closed in the next 7 days unless it is tagged (pinned, good first issue or help wanted) or other activity occurs. Thank you for your contributions.

[github-actions]

This issue has been automatically closed because it has not had activity in the last 37 days. If this issue is still valid, please ping a maintainer and ask them to label it as pinned, good first issue or help wanted. Thank you for your contributions.