Consolidating proto/ and common functions into opentelemetry-common/

[xuan-cao-swi]

Propose to move proto/ and common functions into opentelemetry-common/ so that otlp, otlp-metrics, and otlp-logs exporters
can use the centralized utilities.

1. Current opentelemetry-proto is v1.10, while our proto is still v1.8.0. Centralizing proto to common avoids managing three separate proto/ folders.
2. There are common utilities such as prepare_endpoint, prepare_headers, etc. Consolidating them together prevents issues like fix: URI.join dropping path prefix in logs and metrics exporters #2158.

Other things to consider:
otlp and otlp-http are confusing—people may check otlp-http rather than otlp, and otlp-http still needs to be updated if other otlp exporters change.

1. Rename otlp → otlp-trace (similar to otlp-metrics and otlp-logs); if gRPC support is added in the future, consider adding the gRPC exporter inside otlp-trace/.
2. Add a notice that otlp-http is not maintained and redirect users to otlp.

[thompson-tomo]

I would suggest rather than centralising proto into common we introduce a types-otlp gem which contains just the generated types which is versioned as per the proto used in the generation process. We could then replicate what we do with semconv and raise a pr when a new proto release occurs.

In relation to packages I wouldn't deprecate any but instead focus on having a clear hierarchy. The way I see it is:

otlp # meta package containing all otlp packages
  otlp-http # meta package containing all otlp http packages
    otlp-http-log # implementation of http log exporter
    otlp-http-metric # implementation of http metric exporter
    otlp-http-trace # implementation of http trace exporter
  otlp-grpc # meta package containing all otlp grpc packages