fix: stabilize OpenTelemetry exporter builder API (#2257)

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

Author: zeitlinger

docs/apidiffs/current_vs_latest/prometheus-metrics-exporter-opentelemetry-shaded.txt

@@ -8,27 +8,35 @@
 import java.util.Map;
 import javax.annotation.Nullable;
 
-@StableApi
 public class OpenTelemetryExporter implements AutoCloseable {
   private final MetricReader reader;
 
+  /**
+   * @deprecated This constructor is not part of the stable API. Use {@link #builder()} or {@link
+   *     #builder(PrometheusProperties)} instead.
+   */
+  @Deprecated
   public OpenTelemetryExporter(MetricReader reader) {
     this.reader = reader;
   }
 
+  @StableApi
   @Override
   public void close() {
     reader.shutdown();
   }
 
+  @StableApi
   public static Builder builder() {
     return new Builder(PrometheusProperties.get());
   }
 
+  @StableApi
   public static Builder builder(PrometheusProperties config) {
     return new Builder(config);
   }
 
+  @StableApi
   public static class Builder {
 
     private final PrometheusProperties config;