Rational:

* When the config is reloaded and the provider context is canceled, we need to
  exit the current ZK `TargetProvider.Run` method as a new provider will be
  instantiated.
* In case `Stop` is called on the `ZookeeperTreeCache`, the update/events
  channel may not be closed as it is shared by multiple caches and would
  thus be double closed.
* Stopping all `zookeeperTreeCacheNode`s on teardown ensures all associated
  watcher go-routines will be closed eagerly rather than implicityly on
  connection close events.

Document what ports are scraped by default in k8s example

The Kubernetes pod SD creates a target for each declared port, as documented:

https://prometheus.io/docs/operating/configuration/#pod

> The pod role discovers all pods and exposes their containers as targets. For
> each declared port of a container, a single target is generated. If a
> container has no specified ports, a port-free target per container is created
> for manually adding a port via relabeling.

This results in the default port being the declared port, or no port if none are
declared.

This caused the external label application in the notifier to bleed back
into the rule manager's active alerting elements.

Add maintainers' GitHub usernames to MAINTAINERS.md.

CONTRIBUTING.md instructs people to loop them in using that mechanism,
but nothing lists the right username.

Preserve Alertmanager URLs as *url.URL.

Render a nicer link in the web UI.

Merge 1.6.1 release from 1.6 branch

storage: Don't panic if storage has no FPs even after initial wait

Simplify promlint problems gathering, use protobuf accessors

Cut v1.6.0

* Force buckets in a histogram to be monotonic for quantile estimation

The assumption that bucket counts increase monotonically with increasing
upperBound may be violated during:

  * Recording rule evaluation of histogram_quantile, especially when rate()
     has been applied to the underlying bucket timeseries.
  * Evaluation of histogram_quantile computed over federated bucket
     timeseries, especially when rate() has been applied

This is because scraped data is not made available to RR evalution or
federation atomically, so some buckets are computed with data from the N
most recent scrapes, but the other buckets are missing the most recent
observations.

Monotonicity is usually guaranteed because if a bucket with upper bound
u1 has count c1, then any bucket with a higher upper bound u > u1 must
have counted all c1 observations and perhaps more, so that c  >= c1.

Randomly interspersed partial sampling breaks that guarantee, and rate()
exacerbates it. Specifically, suppose bucket le=1000 has a count of 10 from
4 samples but the bucket with le=2000 has a count of 7, from 3 samples. The
monotonicity is broken. It is exacerbated by rate() because under normal
operation, cumulative counting of buckets will cause the bucket counts to
diverge such that small differences from missing samples are not a problem.
rate() removes this divergence.)

bucketQuantile depends on that monotonicity to do a binary search for the
bucket with the qth percentile count, so breaking the monotonicity
guarantee causes bucketQuantile() to return undefined (nonsense) results.

As a somewhat hacky solution until the Prometheus project is ready to
accept the changes required to make scrapes atomic, we calculate the
"envelope" of the histogram buckets, essentially removing any decreases
in the count between successive buckets.

* Fix up comment docs for ensureMonotonic

* ensureMonotonic: Use switch statement

Use switch statement rather than if/else for better readability.
Process the most frequent cases first.

Added min/max to graph to accommodate constant time series

Vendoring update prior to 1.6 release

This is mostly required to enable summaries without quantiles

storage: Several optimizations of checkpointing

* Test Longer Tests in Travis
Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

* Make test Target Run All Tests

* Add test-short to run short tests

test is running all the tests now as we are running make tests in
CircleCI and I think the base image is shared across Prometheus Org.
Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

* Remove Empty Line
Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

- checkpointSeriesMapAndHeads accepts a context now to allow
  cancelling.

- If a shutdown is initiated, cancel the ongoing checkpoint. (We will
  create a final checkpoint anyway.)

- Always wait for at least as long as the last checkpoint took before
  starting the next checkpoint (to cap the time spending checkpointing
  at 50%).

- If an error has occurred during checkpointing, don't bother to sync
  the write.

- Make sure the temporary checkpoint file is deleted, even if an error
  has occurred.

- Clean up the checkpoint loop a bit. (The concurrent Timer.Reset(0)
  call might have cause a race.)

storage: Recover from corrupted indices for archived series

Add triton zone brand metadata

storage: Guard against a corner case of data corruption

storage: Guard against appending to evicted chunk