`external.label` flag supposed to help to distinguish alert or recording rules
source in situations when more than one `vmalert` runs for the same datasource
or AlertManager.
Uniqueness of rule is now defined by combination of its name, expression and
labels. The hash of the combination is now used as rule ID and identifies rule within the group.
Set of rules from coreos/kube-prometheus was added for testing purposes to
verify compatibility. The check also showed that `vmalert` doesn't support
`query` template function that was mentioned as limitation in README.
* vmalert: Add recording rules support.
Recording rules support required additional service refactoring since
it wasn't planned to support them from the very beginning. The list
of changes is following:
* new entity RecordingRule was added for writing results of MetricsQL
expressions into remote storage;
* interface Rule now unites both recording and alerting rules;
* configuration parser was moved to separate package and now performs
more strict validation;
* new endpoint for listing all groups and rules in json format was added;
* evaluation interval may be set to every particular group;
* vmalert: uncomment tests
* vmalert: rm outdated TODO
* vmalert: fix typos in README
