This PR adds gitea-mixin, configurable Grafana dashboards (and potentially prometheus alerts+recording rules) based on Gitea [metrics](https://docs.gitea.io/en-us/config-cheat-sheet/#metrics-metrics).

The overview dashboard is described using jsonnet and grafonnet library: https://grafana.github.io/grafonnet-lib/

Mixins help to define dashboard and alerts as code so they can be collaboratively improved by the users.

![image](https://user-images.githubusercontent.com/14870891/142862822-fe57b384-c74a-4103-8548-033e92f90751.png)

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## Generate config files

You can manually generate dashboards, but first you should install some tools:

```bash
go get github.com/jsonnet-bundler/jsonnet-bundler/cmd/jb
go get github.com/google/go-jsonnet/cmd/jsonnet
# or in brew: brew install go-jsonnet
```

For linting and formatting, you would also need `mixtool` and `jsonnetfmt` installed. If you
have a working Go development environment, it's easiest to run the following:

```bash
go get github.com/monitoring-mixins/mixtool/cmd/mixtool
go get github.com/google/go-jsonnet/cmd/jsonnetfmt
```

The files in `dashboards_out` need to be imported
into your Grafana server.  The exact details will be depending on your environment.

Edit `config.libsonnet` (for example, list of Gitea metrics to be shown under stats can be adjusted). if required and then build JSON dashboard files for Grafana:

```bash
make
```


For more about mixins, please see:
https://github.com/monitoring-mixins/docs
https://www.youtube.com/watch?v=GDdnL5R_l-Y* add gitea mixin

* remove alerts/rules

* gitea-mixin: add interval factor of 1/2 to remove duplicated change events

* gitea-mixin: fix changes panel, add aggregation interval for changes panel

* gitea-mixin: add totals singlestat

* gitea mixin: switch change graph to timeseries type

* add color overrides for issue labels

* bump grafonnet version

* gitea-mixin: convert graphs to timeseries

* gitea-mixin:  make fmt

* gitea-mixin: add .PHONE in Makefile

* gitea-mixin: add time configration

* gitea-mixin: make fmt and collapse addPanel grid

* gitea-mixin: add static ids for shared panels

* gitea-mixin: add flags showIssuesByRepository, showIssuesByLabel to show/hide corresponding panels

* gitea-mixin: update aggregation interval

* gitea-mixin: update defaults

* gitea-mixin: update panel names

* rename dir to gitea-monitoring-mixin

* gitea-mixin: add gitea_issues_open, gitea_issues_closed metrics

* gitea-mixin: update visible name for datasource

* gitea-mixin: update README

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>