omit pause container network metrics

[sharanyad]

Summary

For awsvpc network mode, we obtain the network metrics of the task's network interface and divide that equally among other containers in the task excluding the pause container

amazon-ecs-agent/agent/stats/engine.go

Line 273 in 2874131

numberOfContainers := len(task.Containers) - 1

But it was observed that pause container was still included in the final network metrics calculation leading to incorrect total task's network stats.
This PR fixes it.

Implementation details

do not add network stats to the container if its the pause container

Testing

manual testing:

before changes:

ContainerName: "~internal~ecs~pause",
          CpuStatsSet: {
            Max: 0,
            Min: 0,
            SampleCount: 20,
            Sum: 0
          },
          MemoryStatsSet: {
            Max: 0,
            Min: 0,
            SampleCount: 20,
            Sum: 0
          },
          NetworkStatsSet: {
            RxBytes: {
              Max: 1558,
              Min: 1438,
              OverflowMax: 0,
              OverflowMin: 0,
              OverflowSum: 0,
              SampleCount: 14,
              Sum: 21212
            },
            RxBytesPerSecond: {
              Max: 5.999998092651367,
              Min: 0,
              SampleCount: 13,
              Sum: 11.999961853027344
            },
            RxDropped: {
              Max: 0,
              Min: 0,
              OverflowMax: 0,
              OverflowMin: 0,
              OverflowSum: 0,
              SampleCount: 14,
              Sum: 0
            },
.....

after changes:

ContainerName: "~internal~ecs~pause",
          CpuStatsSet: {
            Max: 0,
            Min: 0,
            SampleCount: 20,
            Sum: 0
          },
          MemoryStatsSet: {
            Max: 0,
            Min: 0,
            SampleCount: 20,
            Sum: 0
          },
          StorageStatsSet: {
            ReadSizeBytes: {
              Max: 0,
.....

New tests cover the changes: yes

Description for the changelog

Bug - Fix task's network stats by omitting pause container in the network metrics calculation.

Licensing

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