Support trace based linking with APM

[andrewvc]

Today linking between Uptime and APM is done via either URL or APM service.name. The connection would be stronger if we used APM trace headers. Sending those headers would be a simple add to heartbeat and allow for much richer cross-linking between apps. Users would be able to just from a specific failure on heartbeat's side to the trace for that exact transaction

Tasks:

• Add trace fields to heartbeat HTTP headers and documents. (see https://gist.github.com/graphaelli/ae416585c67f9b50f9f326a50ed26c91)
• Ensure the recording bit is set to remove any sampling
• Enhance outbound links to APM in Uptime to use that same data
• Feature to pass in header to Synthetics (implemented using setExtraHTTPHeaders)
• Add synthetics functionality Add support for injecting APM trace details for every run synthetics#245

CC @graphaelli

[paulb-elastic]

Captured in https://github.com/elastic/apm-dev/issues/1024, closing