Fully automate dev setup with Gitpod

This commit implements a fully-automated development setup using Gitpod.io, an
online IDE for GitLab, GitHub, and Bitbucket that enables Dev-Environments-As-Code.
This makes it easy for anyone to get a ready-to-code workspace for any branch,
issue or pull request almost instantly with a single click.

Similar to prometheus/prometheus#7749.

Signed-off-by: Matthias Rampke <mr@soundcloud.com>

.gitpod.Dockerfile

@@ -0,0 +1,6 @@
+FROM gitpod/workspace-full
+
+RUN sudo apt-get update && \
+    sudo apt-get install -y netcat-traditional socat && \
+    sudo apt-get clean && \
+    sudo rm -rf /var/lib/apt/lists/*

.gitpod.yml

@@ -0,0 +1,13 @@
+tasks:
+  - init: go get
+    command: go build . && ./statsd_exporter
+  - command: printf 'Try:\n\n\techo "test.gauge:42|g" | socat - TCP:127.0.0.1:9125\n\n'
+
+ports:
+  - port: 9102
+    onOpen: open-preview
+  - port: 9125
+    onOpen: ignore
+
+image:
+  file: .gitpod.Dockerfile

CONTRIBUTING.md

@@ -16,3 +16,5 @@ Prometheus uses GitHub to manage reviews of pull requests.
   and the _Formatting and style_ section of Peter Bourgon's [Go: Best
   Practices for Production
   Environments](http://peter.bourgon.org/go-in-production/#formatting-and-style).
+
+* [![Gitpod ready-to-code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/prometheus/statsd_exporter)