Update common Prometheus files (prometheus-community#90)

Signed-off-by: prombot <prometheus-team@googlegroups.com>

Workaround for unknown ALERT{} label length (prometheus-community#79)

* hash group labels to get the issue search label

Signed-off-by: Josh Gwosdz <jgwosdz@redhat.com>

* make group label hashing easier to read

and disable errcheck linting for hash.Write call

Signed-off-by: Josh Gwosdz <jgwosdz@redhat.com>

* run go mod tidy

Signed-off-by: Josh Gwosdz <jgwosdz@redhat.com>

* hide label hashing behavior behind flag

Signed-off-by: Josh Gwosdz <jgwosdz@redhat.com>

* ignore never occuring error the go way 💪

Signed-off-by: Josh Gwosdz <jgwosdz@redhat.com>

* use JIRALERT{hash...} issue label to make the origin clear

Signed-off-by: Josh Gwosdz <jgwosdz@redhat.com>

* document -hash-jira-label

Signed-off-by: Josh Gwosdz <jgwosdz@redhat.com>

* document -hash-jira-label changes

Signed-off-by: Josh Gwosdz <jgwosdz@redhat.com>

* adapt tests of prometheus-community#75

Signed-off-by: Josh Gwosdz <jgwosdz@redhat.com>

Fix staticcheck errors.

Add custom field examples to config file.

Fix issue prometheus-community#4 nil pointer dereference in log message.

Add a simple UI.

Update main.Version comment to conform to godoc convention.

Use port 9097, reserved at https://github.com/prometheus/prometheus/w…

…iki/Default-port-allocations

Update README