ingester: support chunk transfers on ingester shutdown.

add a Name() method to the stage interface so that debug logging can show you the name of the pipeline stage which just processed the log
remove some unnecessary logging around fsnotify events we don't care about and saving positions
Make the Processing Log Lines doc a first class citizen, I reference this a lot and currently it's hidden behind 3 clicks

This commit introduces chunk transfers, borrowing the mechanism from
Cortex's implementation: when an ingester is shut down with claim
on rollout enabled, the ingester will find a pending ingester and
transfer all of its chunks to it.

Document centos dep

Signed-off-by: Xiang Dai <764524258@qq.com>

- document centos dep
- add `-y` option

Signed-off-by: Xiang Dai <764524258@qq.com>

* Use prometheus pool for line buffer.

* refactor(querier/ingester): TailRequest Lookback window

Moves the specifications of the Lookback Window out of the
logproto.QueryRequest into it's own type logproto.Lookback.
This is required, because the Lookback Window will be used in the TailRequest
as well.

* feat(querier): parse Lookback from HTTP Request

* feat(logcli): send Lookback Window spec with tail request

* feat(querier): include historic entries in tail mode

Extends tailing by sending a configurable amount of historic entries with
before the live entries. This enables a behaviour that is closer to kubectl logs
-f and docker logs -f.

It is implemented by running a regular Query before subscribing to the ingesters.

* fix: adapt tests to Lookback change

* feat(querier): check all errors to make the linter happy

* fix(logproto): flatten Lookback window spec

Flattens the Lookback window spec into the individual queries

* fix(ingester): adapt test to Lookback flatten

Fixes #774

* rounds nanoseconds boundaries to milliseconds

* convert also store query

moving loki-canary project under loki repo

* Added tail length limit to limit duration of live tailing of logs to 1 hour

* Some code refactoring suggested from PR review for live tailing duration limit

* Fixed error messages in live tailing of logs

* Fixed lint errors

* add benchmark for storage queries
* improve iterator to load only on next
* fix memory retained by lazy chunks
* reverse backward lazy iterator

This commit rolls back the changes to the Makefile and build/Dockerfile
that caused CGO_ENABLED=1 to be present during some builds. This commit
causes the journal support to be disabled in any build produced by make.

Journal support can still be enabled in a manual build:

  go build -o cmd/promtail/promtail cmd/promtail

cleanup rake warnings, bump version

Support for reading systemd journal entries has been added. 
promtail will look for a job in scrape_configs with a journal key 
to activate the journal target. 

If GOOS=linux and CGO_ENABLED=1, promtail will now require 
libsystemd headers to be available for building. If GOOS is not 
linux or CGO_ENABLED is not 1, journal support will be unavailable
and a log message will be printed warning the user that their config 
file has journal tailing configured without it being built into promtail. 

See docs/promtail-examples.md for a concrete example of 
using journal support. 

Other structural changes made: 

  1. Ability for checking if scrape.Config.ServiceDiscoveryConfig is 
     non-zero has been added. 

     This was chosen over making ServiceDiscoveryConfig a pointer
     as yaml.v2 cannot parse an inline struct into a pointer value. 

  2. Updated pipeline logger component name to journal_pipeline and
     file_pipeline for JournalTargetManager and FileTargetManager
     respectively.

  3. The positions file will now store positions as strings instead of 
     integers. Existing positions will be read in properly but written out 
     as strings the next time the positions file is saved. This is done to 
     be able to store the journal cursor, which is a string. The positions 
     API has been updated to support reading in the old integer values 
     and the new string values.