track user-agent or other headers

[sleepylemur]

@msmollin can you list which headers you needed logged here? @cedric I remember there was some desire for a universal sdk tracking solution, but it seems like web and mobile may need to mark requests in different ways. We might be able to have a consistent way of representing it in redshift though.

[msmollin]

For historical sake this is the ticket where we initially discussed this: mapzen/ios#310

[sleepylemur]

Unfortunately a lot of pieces need updating to make this work:

• fastly vcl for tiles and pelias,
• apiaxle-proxy needs to send headers to api-traffic-processors
• api-traffic-processors needs to parse them
• schema update for redshift
• build a report to see the info

Each change is pretty simple, but it's a lot of critical components, so they need to be handled with care.

Also it might make sense to bundle in some other features, like optionally passing api_key as a header.