Migrate to 'next' Prometheus client_golang.

This must only be merged once the new client_golang has been merged
from 'next' to 'master'.
Merge pull request #19 from soundcloud/migration/new-prometheus-client-golang

Migrate to 'next' Prometheus client_golang.
Squashed '_vendor/src/github.com/prometheus/client_golang/' content from commit e1342a3

git-subtree-dir: _vendor/src/github.com/prometheus/client_golang
git-subtree-split: e1342a3d39fe887311c4b4d5ccd6e02c519a1cbd
Merge commit 'cca9d674afd7581ff9625cbed15a23b4429aac07' as '_vendor/src/github.com/prometheus/client_golang'
Differentiate read- and write-clusters

The ability to place a cluster into "write-only mode" is intended to
facilitate operational maneuvers, like re-provisioning a farm. The
intended workflow is:

 - Add a new cluster, in write-only mode
 - Perform a backfill, to populate the new cluster
 - Perform sanity checks, e.g. equal key counts
 - Switch the cluster to normal read+write mode

The new mode prevents farm read strategies from querying what's
effectively a known-bad set of instances while they're spinning up.
Replace all instances of '%s' with %q
Prometheus histograms use configurable max summary age
Tests for ParseFarmString
TestWriteOnly
Merge pull request #20 from soundcloud/write-only

Differentiate read- and write-clusters
pool: fix example in README
Log cluster configuration
prometheus.prefix becomes prometheus.namespace
Oops!
README: fix erroneous claim re: LWW-element-set

Closes #21. Thanks, @rsltrifork, for the eagle eyes!
Remove writeonly mode for a cluster.

It's just not used in the way I thought it would be.
Merge pull request #22 from soundcloud/remove-write-only-mode

Remove writeonly mode for a cluster.
First sketch of Select{Offset,Cursor} split
Work in progress
Work in progress