feat(gatsby): Partially release develop SSR to 5% (#28844)

* feat(gatsby): Partially release develop SSR to 5%

* Fix typescript lint

packages/gatsby/src/services/initialize.ts

@@ -10,8 +10,6 @@ import telemetry from "gatsby-telemetry"
 import apiRunnerNode from "../utils/api-runner-node"
 import handleFlags from "../utils/handle-flags"
 import { getBrowsersList } from "../utils/browserslist"
-import { showExperimentNoticeAfterTimeout } from "../utils/show-experiment-notice"
-import sampleSiteForExperiment from "../utils/sample-site-for-experiment"
 import { Store, AnyAction } from "redux"
 import { preferDefault } from "../bootstrap/prefer-default"
 import * as WorkerPool from "../utils/worker/pool"
@@ -58,25 +56,6 @@ Please give feedback on their respective umbrella issues!
   telemetry.trackFeatureIsUsed(`FastDev`)
 }
 
-if (
-  process.env.gatsby_executing_command === `develop` &&
-  !process.env.GATSBY_EXPERIMENTAL_DEV_SSR &&
-  !isCI() &&
-  sampleSiteForExperiment(`DEV_SSR`, 5)
-) {
-  showExperimentNoticeAfterTimeout(
-    `Server Side Rendering (SSR) in Development`,
-    `gatsby.dev/dev-ssr-feedback`,
-    `which helps surface issues with build errors more quickly. Here's how to try it:
-
-module.exports = {
-  flags : { DEV_SSR: true },
-  plugins: [...]
-}`,
-    1 // Show this immediately to the subset of sites selected.
-  )
-}
-
 // Show stack trace on unhandled promises.
 process.on(`unhandledRejection`, (reason: unknown) => {
   // https://github.com/DefinitelyTyped/DefinitelyTyped/issues/33636

packages/gatsby/src/utils/flags.ts

@@ -1,6 +1,8 @@
 import _ from "lodash"
 import semver from "semver"
 
+import sampleSiteForExperiment from "./sample-site-for-experiment"
+
 // Does this experiment run for only builds
 type executingCommand = "build" | "develop" | "all"
 
@@ -90,9 +92,15 @@ const activeFlags: Array<IFlag> = [
     command: `develop`,
     telemetryId: `DevSsr`,
     experimental: false,
-    description: `SSR pages on full reloads during develop. Helps you detect SSR bugs and fix them without needing to do full builds.`,
+    description: `Server Side Render (SSR) pages on full reloads during develop. Helps you detect SSR bugs and fix them without needing to do full builds.`,
     umbrellaIssue: `https://gatsby.dev/dev-ssr-feedback`,
-    testFitness: (): fitnessEnum => true,
+    testFitness: (): fitnessEnum => {
+      if (sampleSiteForExperiment(`DEV_SSR`, 5)) {
+        return `OPT_IN`
+      } else {
+        return true
+      }
+    },
   },
   {
     name: `QUERY_ON_DEMAND`,