[skip changelog] Gather downloads stats from GitHub releases (#665)

* add stats script

* send metrics

* add failure event and comments

* confusing languages

* testing failures

* revert, failures reporting works ok

* rename file, remove test clause

.github/workflows/github-stats.yaml

+name: download-stats
+
+on:
+  schedule:
+    # run every 5 minutes
+    - cron:  '*/5 * * * *'
+
+jobs:
+  push-stats:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Fetch downloads count
+        id: fetch
+        uses: actions/github-script@0.2.0
+        with:
+          github-token: ${{github.token}}
+          script: |
+            let metrics = []
+
+            // Get a list of releases
+            const opts = github.repos.listReleases.endpoint.merge({
+              ...context.repo
+            })
+            const releases = await github.paginate(opts)
+
+            // Get download stats for every release
+            for (const rel of releases) {
+              // Names for assets are like `arduino-cli_0.4.0_Linux_32bit.tar.gz`,
+              // we'll use this later to split the asset file name more easily
+              const baseName = `arduino-cli_${rel.name}_`
+
+              // Get a list of assets for this release
+              const opts = github.repos.listAssetsForRelease.endpoint.merge({
+                ...context.repo,
+                release_id: rel.id
+              })
+              const assets = await github.paginate(opts)
+
+              for (const asset of assets) {
+                // Ignore files that are not arduino-cli packages
+                if (!asset.name.startsWith(baseName)) {
+                  continue
+                }
+
+                // Strip the base and remove file extension to get `Linux_32bit`
+                systemArch = asset.name.replace(baseName, "").split(".")[0].split("_")
+
+                // Add a metric object to the list of gathered metrics
+                metrics.push({
+                  "type": "gauge",
+                  "name": "arduino.downloads.total",
+                  "value": asset.download_count,
+                  "host": "${{ github.repository }}",
+                  "tags": [
+                    `version:${rel.name}`,
+                    `os:${systemArch[0]}`,
+                    `arch:${systemArch[1]}`,
+                    "cdn:github.com",
+                    "project:arduino-cli"
+                  ]
+                })
+              }
+            }
+
+            // The action will put whatever we return from this function in
+            // `outputs.result`, JSON encoded. So we just return the array
+            // of objects and GitHub will do the rest.
+            return metrics
+
+      - name: Send metrics
+        uses: masci/datadog@v1
+        with:
+          api-key: ${{ secrets.DD_API_KEY }}
+          # Metrics input expects YAML but JSON will work just right.
+          metrics: ${{steps.fetch.outputs.result}}
+
+      - name: Report failure
+        if: failure()
+        uses: masci/datadog@v1
+        with:
+          api-key: ${{ secrets.DD_API_KEY }}
+          events: |
+            - title: "Arduino CLI stats failing"
+              text: "Stats collection failed"
+              alert_type: "error"
+              host: ${{ github.repository }}
+              tags:
+                - "project:arduino-cli"
+                - "cdn:github.com"