Report all web vital metric changes (#949)

With this change, the Web Vital callbacks will be called for all
changes. instead of waiting for them to complete.

See this comment:
If the `reportAllChanges` configuration option is set to `true`, the
`callback` function will be called as soon as the value is initially
determined as well as any time the value changes throughout the page
lifespan.

https://github.com/GoogleChrome/web-vitals/blob/5973d51be39ab0020268aa2397ead0962eedf4c2/src/onCLS.ts#L43-L46

common/js/web_vital_init.js

@@ -13,13 +13,15 @@ function print(metric) {
 }
 
 function load() {
-  webVitals.onCLS(print);
-  webVitals.onFID(print);
-  webVitals.onLCP(print);
-
-  webVitals.onFCP(print);
-  webVitals.onINP(print);
-  webVitals.onTTFB(print);
+  const reportAllChanges = {
+    reportAllChanges: true,
+  }
+  webVitals.onCLS(print, reportAllChanges);
+  webVitals.onFID(print, reportAllChanges);
+  webVitals.onLCP(print, reportAllChanges);
+  webVitals.onFCP(print, reportAllChanges);
+  webVitals.onINP(print, reportAllChanges);
+  webVitals.onTTFB(print, reportAllChanges);
 }
 
 load();