Elastic Agent does not set correct content-type for Fleet Server APIs

[AndersonQ]

For confirmed bugs, please report:

• Version: 8.14.2, main, all with the upload feature
• Operating System: all
• Discuss Forum URL: N/A
• Steps to Reproduce:
  • run a fleet managed agent
  • request a diagnostics upload
  • monitor the traffic
  • check the content type for URLs like /api/fleet/uploads/. It'll be set as application/json

The Elastic Agent does not correctly set the content-type for outgoing requests. It actually has application/json for any request made to Fleet Server:

https://github.com/elastic/elastic-agent/blob/2ac8064fae2e56fcebfd0f7ad5d57bbffb093dce/internal/pkg/remote/client.go#L189-L192

Fleet Server API defines the content-type for the upload API /api/fleet/uploads/{id}/{chunkNum} as */*, not application/json:

https://github.com/elastic/fleet-server/blob/e52dd873b7fa3ef035b45c1b1e14f6ae697db8a1/model/openapi.yml#L1566-L1573

The Elastic Agent should correctly set the content-type of the request it makes to:

• match the API definition
• match the real content type

It's important as customer might have proxies or firewalls that rely or require the content type header to match the actual content type of the request body

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[cmacknz]

We unconditionally set Content-Type to application/json in our Fleet client which is right everywhere except for the Chunk upload endpoint.

https://github.com/elastic/elastic-agent/blob/8e35021ab849470f30d52b623d3af5e4fc682c8e/internal/pkg/remote/client.go#L189-L192