Preserve multiple HTTP header values as a list instead of a joined string

[aabmass]

Right now, HTTP headers are stored as list type span attribute with one item. If there are repeated headers, they are joined with a comma as (being implemented in #2266). We could instead preserve each header value as a separate string.

Current behavior:

{
  "http.request.header.custom_test_header_1": ("test-header-value-1,test-header-value-2",),
}

Proposed behavior

{
  "http.request.header.custom_test_header_1": ("test-header-value-1", "test-header-value-2"),
}

This may require changing some public signatures or adding new overloads to keep backward compatibility.

[aabmass]

Somewhat related, a bunch of signatures in ASGI and WSGI instrumentations are already incorrect #2358

[samuelcolvin]

Fix proposed in #2361.