fix: preserve all headers from response metadata

[lkimtra]

Problem

When forwarding headers from gRPC response metadata to HTTP response,
insert_header was being used which overwrites any existing header
with the same name. This causes data loss when the metadata contains
multiple headers with the same key.

Solution

Changed from insert_header to append_header to preserve all headers
from gRPC metadata exactly as they are, without overwriting duplicates.

Example

Headers like Set-Cookie, Link, Warning, or any custom headers that
may appear multiple times are now correctly forwarded to the HTTP response.