Clarify differences between prometheus protocols, including target info metric naming

[dashpole]

Fixes https://github.com/open-telemetry/opentelemetry-specification/issues/3700

Changes

The current specification mixes OpenMetrics and Prometheus in ways that can be confusing. This change first explains the differences between various Prometheus formats, and then consistently uses "Prometheus" to refer to the general conversion rules. This is helpful for explaining why the target (info) metric naming differs depending on the format used.

Also, fix references to the "target" info-typed metric, which was previously referred to as "target_info". "target_info" is only used when info-typed metrics are not supported.

[dashpole]

@open-telemetry/wg-prometheus

[dashpole]

target metrics would still be called target_info when ingested in prometheus? I.e. this isn't a breaking change, correct?

Correct. This is keeping the same behavior, and is just trying to make the existing spec easier to read.

The only hesitation I have with the current spec is that if Prometheus text format adds an info type, it would be breaking for us to start using it, since we would change the name from target_info to target.