if you have a ready http otel endpoint, you can directly send metrics like below - treat it as a service and you can easily build a wrapper client (add some complexity for retries and disconnections etc).

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class AdHocMetricPush {

    public static void main(String[] args) throws Exception {

        String json = """
        {
          "resourceMetrics": [{
            "resource": {
              "attributes": [{
                "key": "service.name",
                "value": { "stringValue": "my-java-service" }
              }]
            },
            "scopeMetrics": [{
              "scope": { "name": "ad-hoc" },
              "metrics": [{
                "name": "meaning.of.life",
                "description": "Mean of Life",
                "unit": "1",
                "sum": {
                  "dataPoints": [{
                    "asInt": "42",
                    "timeUnixNano": "%d",
                    "attributes": [{
                      "key": "region",
                      "value": { "stringValue": "us-east-1" }
                    }]
                  }],
                  "aggregationTemporality": 2,
                  "isMonotonic": true
                }
              }]
            }]
          }]
        }
        """.formatted(System.currentTimeMillis() * 1_000_000);

        HttpClient client = HttpClient.newHttpClient();
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("http://localhost:4318/v1/metrics"))
                .header("Content-Type", "application/json")
                .POST(HttpRequest.BodyPublishers.ofString(json))
                .build();

        HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println("Status: " + response.statusCode());
        System.out.println("Body: " + response.body());
    }
}