regression in service telemetry metrics configuration

[codeboten]

The following configuration causes errors in the current version in main:

receivers:
  otlp:
    protocols:
      grpc:
        endpoint: localhost:5317
      http:
        endpoint: localhost:5318

exporters:
  debug:
    verbosity: detailed

service:
  telemetry:
    metrics:
      level: detailed
      readers:
        - periodic:
            interval: 10000
            exporter:
              otlp:
                protocol: http/protobuf
                endpoint: localhost:4318
  pipelines:
    metrics:
      receivers: [otlp]
      exporters: [debug]

With this config, if I wait a few seconds, I get this error message (no need to send any telemetry):

2025/02/03 10:13:45 failed to upload metrics: Post "https:///v1/metrics": http: no Host in request URL

Originally posted by @mx-psi in #12252 (comment)

[codeboten]

The issue is that the following code is no longer present in the configuration code:

func normalizeEndpoint(endpoint string) string {
	if !strings.HasPrefix(endpoint, "https://") && !strings.HasPrefix(endpoint, "http://") {
		return "http://" + endpoint
	}
	return endpoint
}

This is causing the URL (localhost:4318) to be parsed incorrectly.