bug: multi-value headers don't propagate to subgraphs correctly

[nmoutschen]

Describe the bug

When sending multiple headers with the same name but different values to the Router, there's only one header entry that remains in the request to the subgraph

To Reproduce
Steps to reproduce the behavior:

1. Setup a test router using the experimental_logging functionality and header propagation set to forward all headers, with a subgraph that logs all requests
2. Send a request with 2 headers using the same name
3. See that the supergraph request headers have 2 headers of the same name
4. See that the subgraph request headers only have 1
5. See that the subgraph only sees one too

Expected behavior

Header propagation should propagate all values for multi-value headers.

Output
If applicable, add output to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.

[garypen]

Probably need to replace insert() with append() here:

router/apollo-router/src/plugins/headers.rs

Line 378 in 65f97e1

headers.insert(rename.as_ref().unwrap_or(named), value.clone());

and here:

router/apollo-router/src/plugins/headers.rs

Line 390 in 65f97e1

headers.insert(name, value.clone());

Need to carefully examine that plugin because there may be other instances where insert() should be append()