[BUG] Otel / data-prepper 404 error with metrics pipelines

Describe the bug
When metrics are exported from OTEL to data-prepper the following 404 error es received:

0tel-collector | 2022-08-12T20:18:18.517Z error exporterhelper/queued_retry.go:183 Exporting failed. The error is not retryable. Dropping data. {"kind": "exporter", "data_type": "metrics", "name": "otlp/http", "error": "Permanent error: rpc error: code = Unimplemented desc = unexpected HTTP status code received from server: 404 (Not Found); transport: received unexpected content-type "text/plain; charset=utf-8"", "dropped_items": 173}

To Reproduce
Steps to reproduce the behavior:

1. I launch data-prepper with OpenSearch as a sink for thre pipelines: "raw-pipeline" ,"service-map-pipeline" and "metrics-pipeline":
   
2. I launch Oel with the following config:
   

For metrics, I've tried the exporters otlp, otlp/http, otlp/2 and otlp/data-prepper. All unsuccesfully.

3. I instrument a Java Application
   

Expected behavior
Metrics traces sould be imported into OpenSearch. Just to mention, the traces pipeline works OK

Environment (please complete the following information):

• OS: Ubuntu 20.04 LTS
• otel/opentelemetry-collector-contrib:latest
• opensearchproject/data-prepper:latest

Start with the data-prepper metrics-pipeline and its OTLP/HTTP ingestion configuration, then reproduce the 404 using the OpenTelemetry Collector setup described in the issue. Compare this with the working traces pipeline and inspect the OpenSearch sink path. Done means metrics are accepted by data-prepper and imported into OpenSearch without the 404 error.