misc: Rename keys leading up to LCP metric

The idea is to align with the name used in web-vitals (LCP in uppercase,
not lcp) and to constrain the custom container in data to metrics
related to Web Vitals.

Do not double encode as JSON, assume that normalization is solved
elsewhere.

packages/apm/src/integrations/tracing.ts

@@ -520,7 +520,7 @@ export class Tracing implements Integration {
       Tracing._forceLCP();
       if (Tracing._lcp) {
         // Set the last observed LCP score.
-        transactionSpan.setData('_sentry_extra_metrics', JSON.stringify({ lcp: Tracing._lcp }));
+        transactionSpan.setData('_sentry_web_vitals', { LCP: Tracing._lcp });
       }
     }
 
@@ -685,7 +685,7 @@ export class Tracing implements Integration {
             elementId: entry.id,
             // @ts-ignore
             elementSize: entry.size,
-            largestContentfulPaint: entry.startTime,
+            value: entry.startTime,
           };
         }
       };