Howdie!

We have some devs who want to be able to trace a request troughout their
applications.

user -> a -> b -> c
|
|-> d -> e

When a user makes a request to "a" uuid is generated inside the app, and
the request to "b" from "a" will set a header(call it WAKAWAKA to uuid),
"b"'s call will passthrough WAKAWAKA to "c" and "d", "d" will passthrough
WAKAWAKA to "e". Etc.

We aggregate all RTR logs into ELK so it would be super helpful to them to
be able to filter on WAKAWAKA and get all the access logs(and app logs
aswell, they mostly use GELF so its easy for them to add whatewher field
they want) from the services involved.

I had a quick peek at the gorouter(
https://github.com/cloudfoundry/gorouter/blob/76668f5818ea8c089ff52a14fcdfbf703c8e8767/access_log/access_log_record.go#L40)
and it seems like this should be a simple PR.

1. To gorouter.yml add
passthrough_headers:
- WAKAWAKA
- X-Random-Header

2. In makeRecord at the bottom add something like(in psuedo)

data = {}
for header in passthrough_headers:
header_val = r.FormatRequestHeader("X-Forwarded-For")
if header_val:
passthrough_headers[header] = header_val

if data:
fmt.Fprintf(b, data.to_stringified_json())

That would yield a log line like
blurgh.dev.cf.private.domain.com - [21/07/2015:10:17:05 +0000] "GET
/statements?ascending=true&since=2015-06-30T14%3A10%3A03.078Z&skipStatementId=30a88204-0779-4385-9859-e4aabd30baf0
HTTP/1.1" 200 0 17 "-" "NING/1.0" 10.230.31.2:46204 x_forwarded_for:"-"
vcap_request_id:1e58195a-cde6-4afd-7f03-43061c9ea91c
response_time:0.004927106 app_id:9784cd03-050d-4b74-9e90-5f17134a3f08
{"WAKAWAKA": "Space is the place", "X-Random-Header": "Once upon a midnight
dreary, while I pondered weak and weary"}

The reason for a stringified JSON is to make it easy to parse with logstash
or other loganalysis tools.

Before I spend time implementing, is this something you would merge?