Metrics API returns empty response until TS process serves a prediction

Context

When the torchserve process initially starts, metrics API endpoints tested return an empty response. While this is a niche case (most likely TS would have served at least 1 prediction before the user calls the metrics APIs), it seems as though the API is broken.

• torchserve version: Installed from source on latest master
• torch version: 1.6.0
• torchvision version 0.7.0
• java version: openjdk-11
• Operating System and version: Ubuntu 18.04

Your Environment

• Installed using source? [yes/no]: Yes
• Are you planning to deploy it using docker container? [yes/no]:N/A
• Is it a CPU or GPU environment?: CPU
• Using a default/custom handler? [If possible upload/share custom handler/model]:No
• What kind of model is it e.g. vision, text, audio?:N/A
• Are you planning to use local models from model-store or public url being used e.g. from S3 bucket etc.? [If public url then provide link.]:N/A
• Provide config.properties, logs [ts.log] and parameters used for model registration/update APIs: Did not use a config.properties (so just the default config)
• Link to your project [if any]:N/A

Expected Behavior

The /metrics endpoint should return the list of metrics whether or not a prediction had been made. The /metrics?name endpoint should return 0s or display that there were no request logged.

Current Behavior

Both endpoints returns an empty response.

Possible Solution

Steps to Reproduce

How to reproduce:

1. Start a new Torchserve process.
2. Run curl http://127.0.0.1:8082/metrics and verify that there is no response.
3. Make a call to a prediction endpoint. e.g. curl http://127.0.0.1:8080/predictions/densenet161 -T kitten.jpg
4. Run curl http://127.0.0.1:8082/metrics again and verify the expected response is returned (the list of metrics).

Same behavior for curl "http://127.0.0.1:8082/metrics?name[]=ts_inference_latency_microseconds&name[]=ts_queue_latency_microseconds" --globoff

Failure Logs [if any]

Note that the metrics API doesn't return results until the first prediction is served. metrics_api_success.txt