fix: stabilize OpenTelemetry exporter builder API

[zeitlinger]

Blocked by #2265 - to make sure that breaking api change is correctly flagged

Summary

• deprecate the OpenTelemetryExporter(MetricReader) constructor instead of treating it as stable API
• make the builder entry points the stable surface for OpenTelemetryExporter
• grandfather the already-accepted constructor removal in the API diff workflow so unrelated PRs stop inheriting the failure on main

Testing

• mise run lint:fix
• mise run build
• ./mvnw -B verify -pl prometheus-metrics-exporter-opentelemetry,prometheus-metrics-exporter-opentelemetry-shaded -am -P 'api-diff,!examples-and-integration-tests' -DskipTests -Dcoverage.skip=true -Dcheckstyle.skip=true -Dwarnings=-nowarn

[github-actions]

⚠️ Breaking API changes detected — maintainer review required

This PR modifies the published API diff for the following module(s):

• prometheus-metrics-exporter-opentelemetry
• prometheus-metrics-exporter-opentelemetry-shaded

The committed API diff contains breaking-change markers for:

• prometheus-metrics-exporter-opentelemetry
• prometheus-metrics-exporter-opentelemetry-shaded

Please review the changes in docs/apidiffs/current_vs_latest/ carefully before approving.