[BUG] Performance Analyzer plugin not reachable at `/_plugins` after following upgrade guide

[jotok]

Describe the bug

We followed the upgrade guide to rename our plugin URIs to appear under the /_plugins resource instead of the /_opendistro resource. See ca460f. However when we perform manual tests, we're not able to reach the plugin via the /_plugins resource. For example, on a docker container running OpenSearch 1.0.0-beta1 (obtained from the artifacts.opensearch.org, if we see the following behavior:

Requeting the old /_opendistro resource:

# curl "localhost:9600/_opendistro/_performanceanalyzer/metrics?metrics=CPU_Utilization&agg=avg&nodes=all"
{"8WVPCeEhTQK9Bl9Jn_qkTw": {"timestamp": 1621627400000, "data": {"fields":[{"name":"CPU_Utilization","type":"DOUBLE"}],"records":[]}}, "umX7FiMWRxqUczmfzvRw_g" :{"timestamp": 1621627400000, "data": {"fields":[{"name":"CPU_Utilization","type":"DOUBLE"}],"records":[]}}}

Requestiong the /_plugins resource:

# curl "localhost:9600/_plugins/_performanceanalyzer/metrics?metrics=CPU_Utilization&agg=avg&nodes=all
<h1>404 Not Found</h1>No context found for request

The Performance Analyzer Agent and perftop make calls to the Performance Analyzer plugin API. We can't update those packages to use /_plugins instead of /_opendistro until this issue is fixed.

To Reproduce
Steps to reproduce the behavior:

1. Clone performance-analyzer (the Performance Analyzer plugin) and the performance-analyzer-rca (the Performance Analyzer agent) into the same directory.
2. In the performance-analyzer-rca run ./gradlew enableRca to spin up a docker cluster running OpenSearch 1.0.0 beta1 and a local build of the Performance Analyzer plugin
3. Connect to the opensearch1 docker container (docker exec -it $container_id bash)
4. Execute the following commands at the container prompt:

• curl "localhost:9600/_opendistro/_performanceanalyzer/metrics?metrics=CPU_Utilization&agg=avg&nodes=all"
• curl "localhost:9600/_plugins/_performanceanalyzer/metrics?metrics=CPU_Utilization&agg=avg&nodes=all

Expected behavior
Both requests result in an HTTP 200 and a response body describing CPU Utilization

Plugins
Please list all plugins currently enabled.

Host/Environment (please complete the following information):

• OS: MacOS
• Catalina
• Docker version 20.10.2, build 2291f61

[dblock]

I would verify that (2) actually runs the new code that includes #14. Either way I think it's a bug in performance-analyzer and should be opened in that repo, but please correct me if I'm wrong.

[CEHENKLE]

Moved from OpenSearch Engine

[sruti1312]

Confirmed that 9600 port _plugins endpoint is working as expected.

Fix: opensearch-project/performance-analyzer-rca#19