Fixed passing down Sentry metadata over GCP metadata

- we should prefer Sentry metadata over GCP metadata because when we're
  doing distributed tracing, we want to see the full request in context
  of the parent instead of creating a new trace

Author: daniellockyer

src/app.ts

@@ -352,9 +352,8 @@ app.use(async (ctx, next) => {
         return Sentry.continueTrace(
             {
                 sentryTrace:
-                    traceId && spanId
-                        ? `${traceId}-${spanId}-1`
-                        : ctx.req.header('sentry-trace'),
+                    ctx.req.header('sentry-trace') ||
+                    (traceId && spanId ? `${traceId}-${spanId}-1` : undefined),
                 baggage: ctx.req.header('baggage'),
             },
             () => {