Fix Use-After-Free bugs in metrics by using std::string

This change updates ephemeral metric fields from `absl::string_view`
to `std::string`.

The metric framework aggregates labels asynchronously. Using a non-owning
`string_view` for dynamically constructed strings (like hostnames or parsed
URIs) leaves dangling pointers when the original string is destroyed.
Replacing them with `std::string` ensures the framework safely owns a copy
of the data.

PiperOrigin-RevId: 886304558
Change-Id: Iaf7a29e09481b8295306cf307637ba1c14022991