If you are using Spring boot 2.1.5.RELEASE then

1. add dependencies actuator and micrometer-prometheus
   

  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
  </dependency>
 <dependency> 
   <groupId>io.micrometer</groupId>
   <artifactId>micrometer-registry-prometheus</artifactId>
 </dependency>

2. add config to enable access to endpoint /actuator/prometheus

management:
  endpoints:
    web:
      exposure:
       include: '*'

3. try to request http://domain:port/actuator/prometheus

EDIT For kubernetes im using kind deployment:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myAppName
spec:
  replicas: 1
  selector:
    matchLabels:
      app: myAppName
  template:
    metadata:
      labels:
        app: myAppName
      annotations:
        prometheus.io/scrape: "true"
        prometheus.io/port: "8091"
        prometheus.io/path: "/actuator/prometheus"
    spec:
      containers:
        - name: myAppName
          image: images.com/app-service:master
          imagePullPolicy: Always
          ports:
            - containerPort: 8091
          env:
            - name: INSTANCE_IP
              valueFrom:
                fieldRef:
                  fieldPath: status.podIP
            - name: SPRING_PROFILES_ACTIVE
              value: "prod"
            - name: CONFIG_SERVER_ADDRESS
              value: "http://config-server:8888"
          livenessProbe:
            failureThreshold: 3
            httpGet:
              path: /actuator/health
              port: 8091
              scheme: HTTP
            initialDelaySeconds: 45
            periodSeconds: 10
            successThreshold: 1
            timeoutSeconds: 5
          readinessProbe:
            failureThreshold: 5
            httpGet:
              path: /actuator/health
              port: 8091
              scheme: HTTP
            initialDelaySeconds: 30
            periodSeconds: 10
            successThreshold: 1
            timeoutSeconds: 5
      nodeSelector:
        servicetype: mvp-cluster

You can scrap metrics from your actuator endpiont in the same way as Prometheus do it. I use OkHttpClient for that. Example of my client:

OkHttpClient client = new OkHttpClient.Builder()
                .retryOnConnectionFailure(false)
                .followRedirects(false)
                .protocols(Collections.singletonList(Protocol.H2_PRIOR_KNOWLEDGE))
                .build();

All settings are optional. But pay attention to the protocol - it should be the same as on your application's server.

After that you need to build url:

String url = "http://localhost:9090/actuator/prometheus?includedNames=<nameOfThePropertyThatYouNeed>";

You can include here more than 1 property:

String url = "http://localhost:9090/actuator/prometheus?includedNames=<propertyNameOne>,<propertyNameTwo>,<propertyNameThree>";

After that you make request:

Request request =new Request.Builder()
                .url(url)
                .get()
                .build();
Response response = client.newCall(request).execute();
String responseBody = response.body().string();

Now you need to parse responseBody. Do it in the way as Prometheus does it and use classes of Prometheus:

InputStream inputStream = new ByteArrayInputStream(responseBody.getBytes())
CollectorPrometheusMetricsWalker walker = new CollectorPrometheusMetricsWalker();
PrometheusMetricsProcessor<MetricFamily> processor = new TextPrometheusMetricsProcessor(inputStream, walker);
processor.walk();
List<MetricFamily> metricList = walker.getAllMetricFamilies();

MetricFamily object stores all metrics with the same name but with different tags. Use metricFamily.getMetrics() to get List<Metric> Use metric.getValue() to get the value of metric.

So far I only used Grafana to read and render data from Prometheus.

With that I have not tested this client, but it seems there are more than none out there.

Otherwise, it should be quite straightforward to run HTTP GET requests using HttpClient, then parsing the result using Genson.

Could you please clarify: "instrumenting" usually means exposing metrics from your application's code to a Prometheus endpoint. The question reads that you either want to read metrics from an application, or from a Prometheus instance that has already scraped the metrics.

For this answer I assume that you're running a Prometheus instance that has already scraped the metrics.

If you want to read the most current values that Prometheus has scraped from your applications, you can use Prometheus' federation endpoint via HTTP: you can read all metrics with their current readings in one go, or apply a query. I'm not aware of a Java library to parse the format, but you find the definition here. You could use the same approach to query your applications directly.

If you want to receive a JSON that might be easier to parse, you can use Prometheus's HTTP API.

If you want to receive updates on values as soon as Prometheus queries them, you can hook up to the remote write API. There is already existing integrations, but at first glance there is no Java integration. You could use Kafka as an intermediary. Also, this might be more than you've asked for in this questions.