Authentication broken in confighttp.ServerConfig since OTel Collector v0.106.0

[joschi]

Describe the bug

Since OpenTelemetry Collector v0.106.0, receivers based on HTTP using authentication (via auth.authenticators) are failing to parse their configuration with this error message:

decoding failed due to the following error(s):
        	           	
error decoding '': decoding failed due to the following error(s):
        	           	
'protocols.http.auth' has invalid keys: authenticator

Steps to reproduce

This is a small diff on top of v0.106.0 and we see the same issue in a custom component of ours which is using confighttp.ServerConfig (go.opentelemetry.io/collector/config/confighttp).

OpenTelemetry Collector v0.105.0 is working fine.

diff --git receiver/otlpreceiver/testdata/config.yaml receiver/otlpreceiver/testdata/config.yaml
index 018f8d664..258893416 100644
--- receiver/otlpreceiver/testdata/config.yaml
+++ receiver/otlpreceiver/testdata/config.yaml
@@ -26,6 +26,8 @@ protocols:
         min_time: 10s
         permit_without_stream: true
   http:
+    auth:
+      authenticator: nop
     # The following entry demonstrates how to specify TLS credentials for the server.
     # Note: These files do not exist. If the receiver is started with this configuration, it will fail.
     tls:

What did you expect to see?

The tests in receiver/otlpreceiver should be successful.

What did you see instead?

❯ make test
go test -race -timeout 240s ./...
?   	go.opentelemetry.io/collector/receiver/otlpreceiver/internal/metadata	[no test files]
--- FAIL: TestUnmarshalConfig (0.01s)
    config_test.go:85: 
        	Error Trace:	/Users/joschi/src/opentelemetry-collector/receiver/otlpreceiver/config_test.go:85
        	Error:      	Received unexpected error:
        	           	decoding failed due to the following error(s):
        	           	
        	           	error decoding '': decoding failed due to the following error(s):
        	           	
        	           	'protocols.http.auth' has invalid keys: authenticator
        	Test:       	TestUnmarshalConfig
    config_test.go:86: 
        	[...]
FAIL
FAIL	go.opentelemetry.io/collector/receiver/otlpreceiver	2.235s
ok  	go.opentelemetry.io/collector/receiver/otlpreceiver/internal/errors	(cached)
ok  	go.opentelemetry.io/collector/receiver/otlpreceiver/internal/logs	(cached)
ok  	go.opentelemetry.io/collector/receiver/otlpreceiver/internal/metrics	(cached)
ok  	go.opentelemetry.io/collector/receiver/otlpreceiver/internal/trace	(cached)
FAIL
make: *** [test] Error 1

What version did you use?

https://github.com/open-telemetry/opentelemetry-collector/releases/tag/v0.106.0

Additional context

From the release notes of OpenTelemetry Collector v0.106.0 it looks like this change could be causing the issue:

#10445