[BUG] Fix Server-Timing key formatting

[johnboxall]

In #1895, we added the Server-Timing header to HTTP responses.

https://developer.salesforce.com/docs/commerce/pwa-kit-managed-runtime/guide/debugging.html#__server_timing

To namespace metric names we used colons like ssr:route-matching.

It seems like colons (eg. :) are not considered valid characters:

https://www.w3.org/TR/server-timing/#the-server-timing-header-field

The result is that tools like Chrome cannot parse our Server-Timing response:

We should consider using a different character for namespacing and verify that it works in Chrome and other Server-Timing parsing tools.