Guard against tags being nil in EC2 discovery

Fixes #3001

Import documentation

In order to provide documentation for each individual version, this
commit starts moving Prometheus server specific documentation into the
repository itself.

If the other Prometheus has an external label that matches that of
the Prometheus being read from, then we need to remove that matcher
from the request as it's not actually stored in the database - it's
only added for alerts, federation and on the output of the remote read
endpoint.
Instead we check for that label being empty, in case there is a time
series with a different label value for that external label.

Fixes #3261

Cut v1.8.0

Only respond to API requests once the server is ready

Avoid blocking in the logThrottling loop

The timer semantics is really hard. The simple pattern as given in the
godoc for the time package assumes we are not elsewhere consuming from
the timer's channel. However, exactly that can happen here with the
right sequence of events. Thus, we have to drain the channel only if
it has something to drain.

[storage/local] fix timer.Reset usage

This was a warning and can be a frequent occurrence.  Let's not fill up
logs unless we are asked to.

According to https://golang.org/pkg/time/#Timer.Reset you must not rely
on a returned value from Reset.
Signed-off-by: NAlexander Morozov <lk4d4math@gmail.com>

Removes configurability introduced in #3160 in favour of hard-coding,
per advice from @brian-brazil.

cmd/prometheus: remove govalidator for url validation

This prevents accidental copy/paste error where a the web.external-url
or alertmanager.url params could have an extra set of quotes.
See also: https://github.com/prometheus/prometheus/issues/1229

The usage of govalidator is redundant with the call to url.Parse for
url validation. Removing it has the following benefits:

 - The explicit error message is displayed instead of just a generic
   valid/invalid message
 - Slightly smaller code with one fewer external dependency
 - Speed improvement by removing duplicate call to url.Parse (inside
   govalidator.IsURL()
 - Resolves issue #2717

The only potential drawback of removing govalidator is that certain
URLs will be considered valid which were previously invalid. For example:

 - URLs with hostnames that start and/or end with an underscore (http://_example.com_)
 - URLs with hostnames that contain some special characters (http://foo&*bar.org)

These are valid URIs according to RFC 3986 and valid domain names per RFC 2181,
however they are not valid hostnames per RFC 952.

Implement remote read server in Prometheus.

Move to Go1.9

Cut v1.7.2

util/stats: remove dead code

config: simplify some returns

Make all rule links link to the "Console" tab rather than "Graph"

Clicking on a rule, either the name or the expression, opens the rule
result (or the corresponding expression, repsectively) in the
expression browser. This should by default happen in the console tab,
as, more often than not, displaying it in the graph tab runs into a
timeout.