As mentioned in https://grafana.com/docs/loki/latest/operations/authentication/#manage-authentication, Loki does not have any built-in conventions for authentication mechanisms, so the request header used for authentication can be quite flexible; it does not necessarily be Authorization: XXX.
This is different from clickhouse-logger and elasticsearch-logger, which have built-in authentication.

This is exactly why I didn't add any authentication to the current plugin.

Therefore, it is better to design a generic mechanism for it, i.e., provide a user-configurable object to store one or more request headers.

I don't have any idea about the name of the field, it could use headers, request_headers or auth_headers, ask other reviewers to take a look.

IMHO, this tests nothing. There isn't even a validation to prove that the header actually sends to the configured endpoint, it's too perfunctory.

Use the /echo endpoint to validate the request header, which is sent back as a response header.

Sorry, there is no local setup guide, and I cannot run make init and the local tests successfully, I only conducted actual manual testing.
And I don’t know how to use /echo to verify requests sent to the Loki URL.

Then, please do this:

1. Create route with the following upstream and URI

=== TEST XX: setup route (test auth headers)
--- config
    location /t {
        content_by_lua_block {
            local t = require("lib.test_admin").test
            local code, body = t('/apisix/admin/routes/1',
                ngx.HTTP_PUT,
                [[{
                    "plugins": {
                        "loki-logger": {
                            "endpoint_addrs": ["http://127.0.0.1:1980"],
                            "endpoint_uri": "/log_request",
                            "request_headers": {"Authorization": "test1234"},
                            "batch_max_size": 1
                        }
                    },
                    "upstream": {
                        "nodes": {
                            "127.0.0.1:1980": 1
                        },
                        "type": "roundrobin"
                    },
                    "uri": "/hello"
                }]]
            )

            if code >= 300 then
                ngx.status = code
            end
            ngx.say(body)
        }
    }
--- response_body
passed

2. Send a request to /hello endpoint to trigger the log, then check the error.log.

=== TEST XX+1: hit route
--- request
GET /hello
--- response_body
hello world
--- error_log
go(): authorization: test1234

The /log_request endpoint used by loki will log the following error.log.

It contains every request header, which obviously includes the one we configured for loki for authentication.