Add observability to asset-worker outer entrypoint RPC methods

[courtney-sims]

Fixes #[insert GH or internal issue link(s)].

Describe your change...

The outer entrypoint's RPC methods (unstable_canFetch, unstable_getByETag, unstable_getByPathname, unstable_exists) previously had no error reporting or tracing. That was only configured for the main fetch. When the router-worker called the RPC methods and they threw, the error was invisible.
This was discovered when deploying the loopback changes to staging. Both flamingo edge tests failed, but nothing appeared in Sentry.

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because: observability changes only
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: internal changes only

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Latest commit: a80ca4f

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commit: a80ca4f