Feature #18 add x flow

[lmineiro]

The implementation generates a random string with 8 to 64 bytes and sets a HTTP header X-Flow-Id that other systems can use to correlate log entries.
The filter accepts some parameters that help control the its behavior, namely, to accept an existing flow id and the length of the generated flow ids
This closes #18

[aryszka]

no need for the braces when there is only a single var, but of course fine like this, too

[lmineiro]

I believe we can add some more global errors soon :)

[aryszka]

we don't have bool :(
the doc is not yet in the master, but you can have a look at here:
https://github.com/zalando/skipper/blob/release-cleanup-doc/eskip/doc.go

[mo-gr]

Have you considered just having an array of valid chars and then len times pulling a random index out of that one. Seems to me, that this might be faster than crypto-random and hex operations

[lmineiro]

Pre-defining an alphabet seems like a good idea. I'm sure it will be faster and, you're right, for this we don't need crypto-randomness.

[lmineiro]

$ curl -k -X PUT -d 'value=Path("/flowid") -> flowId() -> "http://localhost:8080"' \
http://127.0.0.1:2379/v2/keys/skipper/routes/testFlowId

---

$ curl -v http://localhost:9090/flowid
* Hostname was NOT found in DNS cache
*   Trying ::1...
* Connected to localhost (::1) port 9090 (#0)
> GET /flowid HTTP/1.1
> User-Agent: curl/7.38.0
> Host: localhost:9090
> Accept: */*

---

$ nc -l 8080
GET /flowid HTTP/1.1
Host: localhost:8080
User-Agent: curl/7.38.0
Accept: */*
X-Flow-Id: 4vI2AarIh5hqqsuD
Accept-Encoding: gzip

[aryszka]

lgtm, very nice

[danpersa]

new instead of make? shouldn't we be consistent with naming?

[lmineiro]

Yes, we should. It will be New*(). Refactoring the old filters will be next

[aryszka]

it's already in progress to switch to new, see the release-cleanup branch

[danpersa]

lgtm