ci: Enable coverage on main and all PRs (#2533)

In preparation for some larger changes to the proxy, this change enables
codecov on main and on PRs that change code. We need to run coverage on
main so that PRs have a meaningful comparison point.

We use the styfle/cancel-workflow-action to prevent multiple runs
from clogging up Actions Runners.

We can adapt this configuration if it's too annoying, but it seems low
risk to add for now.

.codecov.yml

@@ -1,8 +1,34 @@
+codecov:
+  ignore:
+    - "linkerd/app/integration/src/**"
+    - "**/src/gen/**"
+    - "tools/**"
+
 coverage:
-  # Disable statuses until we sort out coverage more broadly. This is noisy as
-  # it is.
+  precision: 2
+  round: down
+  range: "70...100"
+
   status:
     project:
-      default: false
+      default:
+        target: auto
+        threshold: 2%
+        if_not_found: success
+        if_ci_failed: error
+        paths:
+          - "!linkerd/app/integration/src/**"
+          - "!**/src/gen/**"
+          - "!tools/**"
+
     patch:
-      default: false
+      default:
+        target: auto
+        threshold: 1%
+        if_not_found: success
+        if_ci_failed: error
+
+comment:
+  layout: "header, diff, files, footer"
+  behavior: default
+  require_changes: false

.github/workflows/coverage.yml

@@ -4,13 +4,18 @@ permissions:
   contents: read
 
 on:
+  # We run coverage on main so that PRs can have a meaninful comparison that
+  # doesn't lag a whole day.
+  push:
+    branches:
+      - main
+
+  # Coverage runs on all code changes (as well as changes to the workflow).
   pull_request:
     paths:
       - .codecov.yml
       - .github/workflows/coverage.yml
-  schedule:
-    # Run daily at 11AM UTC (3AM PST).
-    - cron: '0 11 * * *'
+      - '**/*.rs'
 
 env:
   CARGO_INCREMENTAL: 0
@@ -19,6 +24,18 @@ env:
   RUSTUP_MAX_RETRIES: 10
 
 jobs:
+  # Cancel any previous runs of this workflow so coverage runs don't back up,
+  # especially on main (i.e. after merging several PRs).
+  cleanup:
+    runs-on: ubuntu-latest
+    permissions:
+      actions: write
+    steps:
+      - uses: styfle/cancel-workflow-action@01ce38bf961b4e243a6342cbade0dbc8ba3f0432
+        with:
+          all_but_latest: true
+          access_token: ${{ github.token }}
+
   test:
     name: codecov
     runs-on: ubuntu-latest