Add mem_shared metric to CloudWatch Agent

[ola-amr]

Description

This PR implements the mem_shared metric as a configuration option for the Agent to collect from the Telegraf plugin.

Description of changes

• Added shared to the list of registered metrics for linux servers running the agent.
• Set the shared metric's units to bytes
• Added mem_shared to validLinuxMetrics.json for schema validation

Testing

• Deployed a local version of the CloudWatch agent with my changes on an EC2 instance
• Configured agent with the following configuration:

{
  "metrics": { 
    "namespace": "mem_shared_testing",
    "metrics_collected": {
      "mem": {
        "measurement": [
          "mem_shared"
        ],
        "metrics_collection_interval": 10
      }
    }
  }
}

• Verified metrics appearing correctly in CloudWatch console

• Validated metric accuracy by comparing with direct system values:

  $ cat /proc/meminfo | grep -i shm
  Shmem:               184 kB

• Confirmed shared memory value (184 kB) matches between system and emitted metric