Add retries to get ec2 instance-id

[mythri-garaga]

Summary

Currently when agent fails to get instance id from EC2 metadata service, agent overwrites InstanceID in state file with ''. This PR includes changes to call instance metadata service at least 3 times to get instance id before returning an empty string.

level=critical time=2021-03-04T00:00:05Z msg="Error saving state before final shutdown: Multiple error:\n\t0: final save: timed out trying to save to disk" module=termination_handler.go
level=info time=2021-03-04T00:00:07Z msg="Loading configuration" module=agent.go
level=info time=2021-03-04T00:00:07Z msg="Amazon ECS agent Version: 1.50.0, Commit: 90ccb1d4" module=agent.go
level=info time=2021-03-04T00:00:07Z msg="Image excluded from cleanup: amazon/amazon-ecs-pause:0.1.0" module=docker_image_manager.go
level=info time=2021-03-04T00:00:07Z msg="Image excluded from cleanup: amazon/amazon-ecs-pause:0.1.0" module=docker_image_manager.go
level=info time=2021-03-04T00:00:07Z msg="Image excluded from cleanup: amazon/amazon-ecs-agent:latest" module=docker_image_manager.go
level=info time=2021-03-04T00:00:07Z msg="Creating root ecs cgroup: /ecs" module=init_linux.go
level=info time=2021-03-04T00:00:07Z msg="Creating cgroup /ecs" module=cgroup_controller_linux.go
level=info time=2021-03-04T00:00:07Z msg="Event stream ContainerChange start listening..." module=eventstream.go
level=warn time=2021-03-04T00:00:07Z msg="Unable to access EC2 Metadata service to determine EC2 ID: RequestError: send request failed\ncaused by: Get \"http://169.254.169.254/latest/meta-data/instance-id\": EOF" module=agent.go
level=warn time=2021-03-04T00:00:07Z msg="Data mismatch; saved InstanceID 'i-xxxxxxxxx' does not match current InstanceID ''. Overwriting old datafile" module=agent.go
level=info time=2021-03-04T00:00:08Z msg="Registering Instance with ECS" module=agent.go

Implementation details

• In `./agent/app/agent.go - Add retries with exponential back off when calling IMDS for ec2 instance id
• In `./agent/app/agent_test.go - Updated tests accordingly

Testing

make test passes
New tests cover the changes: yes

Description for the changelog

Enhancement - Add retries while retrieving instance-id using EC2 Instance metadata service api

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.