Update Metadata payload to conform to ELK 7.x JSON schema

[linuxwebexpert]

|#217 - https://projects.vta.org/projects/rti-bounce-proxy/work_packages/251/activity

After having attended Elastic APM workshop, it seems the community needs some help if we are to get our issues of defective Moovbox hardware resolved. So far we've tested the current 6.7.x branch of the parent fork and found it partially usable. Seems there has been lots of changes in the ELK API structure, including the deprecated aka deleted v1/transactions API endpoint.

So to make a v2 compatible APM agent, we need to send validated NDJSON based on Elastic's latest implementation standards and JSON schema documentation, please see example here @ https://www.elastic.co/guide/en/apm/server/7.0/metadata-api.html

• Create sample file output for debugging using Elastic 7.x schema files
• Import APM Server docs/spec JSON schema for validation
• Implement jshint validation script for sample data and JSON schemas
• Develop PHP code to properly send the metadata schema to Elastic APM endpoint