Enable X-Ray tracing

[Marco Lüthy]

~/serverless.yml:

    # Enable X-Ray tracing on Lambda functions
    SharpLambdaFunction:
      Properties:
        TracingConfig:
          Mode: Active

• attach AWSXrayWriteOnlyAccess IAM policy.
• instrument AWS SDK calls (more)

var AWSXRay = require('aws-xray-sdk-core');
var AWS = AWSXRay.captureAWS(require('aws-sdk'));