Added KES grafana dashboard

[aead]

LGTM @shtripat - I just want to change the location where things are stored.

Can we move:

• docs/metrics/prometheus/grafana/grafana-kes.png => .github/grafana-dashboard.png
• docs/metrics/prometheus/grafana/kes-dashboard.json => examples/grafana/dashboard.json

And instead of a separate README, just add a sub-section to the top level README Docs:

#### Monitoring

KES servers provide an API endpoint `/v1/metrics` that observability tools, like Prometheus, can scrape.
Refer to the [monitoring documentation](https://min.io/docs/kes/concepts/monitoring/) for how to setup and capture KES metrics.

For a graphical Grafana dashboard refer to the following [example](examples/grafana/dashboard.json).

![.github/grafana-dashboard.png]

[aead]

cc @djwfyi - We may want to update our KES monitoring docs to include the grafana dashboard