Plan: Continuous perf testing

# Continous performance testing for CoreDNS

## Setup

drone.coredns.io (Packet machine) runs [`webhook`](https://github.com/adnanh/webhook/) behind a Caddy server. Caddy is used for Let's Encrypt TLS certs and only proxies.

Further more [mbench](https://github.com/miekg/mbench) is running to collect performance data (`go test -bench=.` output) and convert into prometheus metrics.

Prometheus is running locally, scrapping mbench.

Grafana is running locally displaying graphs from Prometheus.

Of all these things, only Caddy and mbench are *not* installed from the Debian repos.

## Integration with Github

Webhook setup on github.com point to drone.io port.

### Trigger a build from Github

On a PR add a comment: `@mbench OK ....`. Only the OK is needed. Only some people
can trigger these.

### Trigger locally

Send a REST request to another endpoint (blocks everything execpt ::1 and 127.0.0.1).

### Master branch

A local cron job trigger the unauthenticated local endpoint every 30 minutes.
## Future

* Each middleware should have more benchmarks
* Publish this data somewhere?

No repository files or tests are named. Start by inspecting the existing shell scripts and the drone/webhook, mbench, Prometheus, and Grafana setup described in the issue. Clarify the benchmark coverage, trigger flow, and destination for published data; done requires an agreed scope and implementation plan.

Written by the indexing model from the issue text.