Hope Service.Meta to contain a special characters such as dot(.)

## Feature Description
I encountered an exception OperationException(statusCode=400, statusMessage='Bad Request', statusContent='Invalid Service Meta: Couldn't load metadata pair ('user.name', 'spring_boot'): Key contains invalid characters') that was related to that issue #4422. I actually want to use Service.Meta to pass come configurations containing a special characters . such as user.name or user.password. Could you relax the restriction about Service.Meta?

#### Use Case(s)
I use [spring boot admin](https://github.com/codecentric/spring-boot-admin) in which BasicAuthHttpHeaderProvider requires the metadata(user.name and user.password) that can not be supported by consul. This project support many discovery client such as eureka, kubernetes service that all support to use `user.name` for metadata. In SpringFramework's perspective, metadata won't be related with DNS so I suggest that `Service.Meta` should keep relaxing.

Start by tracing Service.Meta validation and reviewing the related issue #4422. Determine the intended character rules and compatibility impact, then verify that metadata keys such as user.name and user.password are accepted without breaking existing validation behavior.

Written by the indexing model from the issue text.