Fix #11457: Use proxy-defaults protocol if service-defaults protocol is unset

[Mongey]

Description

Currently if you set a proxy-default to http, and then attempt to create a service-router for a service, without the protocol set, you'll get a 500 error.

discovery chain "foo" uses a protocol "tcp" that does not permit advanced routing or splitting behavior:

See #11457 for more details.

Testing & Reproduction steps

Reproduction steps are in #11457

before:

> consul config write bug-11457/proxy-defaults.hcl;  consul config write bug-11457/service-router.hcl;  consul config write bug-11457/service-defaults.hcl

Config entry written: proxy-defaults/global
Config entry written: service-defaults/foo
Error writing config entry service-router/foo: Unexpected response code: 500 (discovery chain "foo" uses a protocol "tcp" that does not permit advanced routing or splitting behavior)

after:

> consul config write bug-11457/proxy-defaults.hcl;  consul config write bug-11457/service-router.hcl;  consul config write bug-11457/service-defaults.hcl

Config entry written: proxy-defaults/global
Config entry written: service-defaults/foo
Config entry written: service-router/foo

Links

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern