[Benchmarks] Comment on PRs with benchmark scores

This should always leave a comment with a summary of benchmark results. Furthermore, if the alert threshold is triggered then it will leave another comment making it clear that this has happened.

I fully expect that we'll want to disable at least one of these, and possibly tune the alert threshold. But by starting with too much, it will be fast to see and back this off to a useful level.

.github/workflows/benchmark-go.yml

@@ -4,6 +4,9 @@ on:
   push:
     branches:
       - main
+  pull_request:
+    branches:
+      - main
 
 permissions:
   # deployments permission to deploy GitHub pages website
@@ -37,5 +40,6 @@ jobs:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           auto-push: true
           alert-threshold: "150%"
-          comment-on-alert: true
           fail-on-alert: true
+          comment-on-alert: true # notify on PR if alert triggers
+          summary-always: true # always comment on PRs to leave job summary