HCM route request_headers_to_add cannot evaluate upstream host info correctly

[lambdai]

UPSTREAM_METADATA is referring to the metadata in HostDescription, which is reasonable IFF
an upstream host is selected.
However, the evaluation is invoked before the upstream is selected so we can never encode the upstream
info into the header to upstream.

Approach:
Instead run header evaluation during route decoding the downstream header, push back to onPoolReady()
beside the parent_.downstreamHeaders()->setHost(host->hostname());

I evaluate twice which is not ideal but it solve my problem.

+++ b/source/common/router/upstream_request.cc
@@ -378,6 +378,9 @@ void UpstreamRequest::onPoolReady(
     parent_.downstreamHeaders()->setHost(host->hostname());
   }
 
+  parent_.routeEntry()->finalizeRequestHeaders(*headers, parent_.callbacks()->streamInfo(), <Expose insert_envoy_original_path flag to RouteInterface>);

Example config:

    - filters:                                                                                                                                                  
      - name: envoy.http_connection_manager                                               
        config:                                                                           
          codec_type: auto                                                                
          stat_prefix: tls_back                                                           
          route_config:                                                                   
            name: local_route                                                             
            virtual_hosts:                                                                
            - name: backend                                                               
              domains:                                                                    
              - "*"                                                                       
              request_headers_to_add:                                                                                                                                                 
              - header:                                                                                                                         
                  key: X-istio-original-port                                                                                                                                        
                  value: "%UPSTREAM_METADATA([\"istio\", \"port\"])%"                                                                                           
                append: true    

  - name: echo_cluster
    connect_timeout: 1s
    type: strict_dns
    lb_policy: round_robin
    load_assignment:
      cluster_name: "echo_cluster"
      endpoints:
        lb_endpoints:
        - endpoint:
            address:
              socket_address:
                address: 127.0.0.1
                port_value: 15008
          metadata:
            filter_metadata:
              istio:
                port: "80"

[lambdai]

I moved finalizeRequestHeaders to this place. all test cases except 2 cases expect the function be invoked there failed, which is not hard to update test cases.

Please let me know if you have any concerns on move the finalizeRequestHeaders which is not captured by the tests

[zuercher]

I think I'd want to see the PR to understand better the effect of the change.

[stale]

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

[stale]

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 "help wanted". Thank you for your contributions.