Add an option to prefix an endpoint path with an environment variable. (#153)

* Add an option to prefix an endpoint path with an environment variable.

* prefiv environment variable with next_public_ so it gets inlined at build time

* correct env name everywhere, and make more specific

* chore: use correct name

---------

Co-authored-by: feugy <damien@vercel.com>

Author: emspishak

packages/web/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@vercel/analytics",
-  "version": "1.4.0",
+  "version": "1.5.0-canary.1",
   "description": "Gain real-time traffic insights with Vercel Web Analytics",
   "keywords": [
     "analytics",

packages/web/src/generic.ts

@@ -20,6 +20,8 @@ export const DEV_SCRIPT_URL =
   'https://va.vercel-scripts.com/v1/script.debug.js';
 export const PROD_SCRIPT_URL = '/_vercel/insights/script.js';
 
+export const basepathVariableName = 'NEXT_PUBLIC_WEB_ANALYTICS_BASEPATH';
+
 /**
  * Injects the Vercel Web Analytics script into the page head and starts tracking page views. Read more in our [documentation](https://vercel.com/docs/concepts/analytics/package).
  * @param [props] - Analytics options.
@@ -67,6 +69,8 @@ function inject(
   }
   if (props.endpoint) {
     script.dataset.endpoint = props.endpoint;
+  } else if (process.env[basepathVariableName]) {
+    script.dataset.endpoint = `/${process.env[basepathVariableName]}/_vercel/insights`;
   }
   if (props.dsn) {
     script.dataset.dsn = props.dsn;