[exporter/awsxray] Allow multiple log group names/arns to be set in environmental variables

[pxaws]

Description: <Describe what has changed.>

Cherry-picking from downstream:

amazon-contributing#218

• We allow users to set multiple log group names or arns using environmental variable. We choose to use & to separate individual log group name or arn because & is not allowed in log group names (https://docs.aws.amazon.com/AmazonCloudWatchLogs/latest/APIReference/API_CreateLogGroup.html) and & implicitly means 'AND'.
• This PR adds the parsing logic in xray exporter to handle multiple log group names/arns set in environmental variable.

Link to tracking Issue: <Issue number if applicable> N/A

Testing: <Describe what testing was performed and which tests were added.> Unit Testing

Documentation: <Describe the documentation added.> The readme for xray exporter is updated.