Document how to define a custom meter registry (#112)

Adds a documentation section explaining how Micrometer users can create their own MeterRegistry implementation.

Closes gh-111

src/components/DocRoot/index.js

@@ -160,6 +160,9 @@ export default function DocRoot() {
           <ul>
             <li><NavLink className="doc-section" to="/docs/guide/httpSenderResilience4jRetry">HttpSender with Resilience4j retry</NavLink></li>
           </ul>
+          <ul>
+            <li><NavLink className="doc-section" to="/docs/guide/customMeterRegistry">Custom meter registry</NavLink></li>
+          </ul>
         </li>
         <li>
           <NavLink className="doc-section" to="/docs/support">Support policy</NavLink>. Micrometer's support policy for releases.

src/components/DocRoutes/index.js

@@ -14,6 +14,7 @@ let docsOkHttpClient = require('!asciidoc-loader!../../docs/okhttpclient/index.a
 let docsConsoleReporter = require('!asciidoc-loader!../../docs/guide/console-reporter.adoc');
 let docsHealthCheck = require('!asciidoc-loader!../../docs/guide/health-check.adoc');
 let docsHttpSenderResilience4jRetry = require('!asciidoc-loader!../../docs/guide/http-sender-resilience4j-retry.adoc');
+let docsCustomMeterRegistry = require('!asciidoc-loader!../../docs/guide/custom-meter-registry.adoc');
 let docsSupport = require('!asciidoc-loader!../../docs/support/index.adoc');
 
 const systems = ['appOptics', 'atlas', 'azure-monitor', 'datadog', 'dynatrace', 'elastic', 'ganglia', 'graphite', 'humio', 'influx', 'instana', 'jmx', 'kairos', 'new-relic', 'prometheus', 'signalFx', 'stackdriver', 'statsD', 'wavefront'];
@@ -67,6 +68,10 @@ export default function DocRoutes() {
         <DocSection title="HttpSender with Resilience4j retry" content={docsHttpSenderResilience4jRetry}/>
       }/>
 
+      <Route path="/docs/guide/customMeterRegistry" render={() =>
+        <DocSection title="Custom meter registry" content={docsCustomMeterRegistry}/>
+      }/>
+
       <Route path="/docs/support" render={() =>
         <DocSection title="Micrometer Support Policy" content={docsSupport}/>
       }/>

src/docs/guide/custom-meter-registry.adoc

@@ -0,0 +1,72 @@
+= Custom meter registry
+
+Micrometer supports popular meter registries out of the box, so please check them first.
+For an existing meter registry, if you think that your requirements are generally useful, please consider creating an issue or PR against the Micrometer issue tracker.
+For a non-existing meter registry, if it's widely-used, please consider creating an issue or PR for it.
+
+For some reasons such as unusual requirements, if you want to create your own custom meter registry, you can create it by extending one of base classes for meter registries: `MeterRegistry`, `PushMeterRegistry`, and `StepMeterRegistry`.
+
+Most common way is to extend `StepMeterRegistry`.
+You can create your own custom `StepMeterRegistry` as follows.
+
+First, define your own meter registry configuration by extending `StepRegistryConfig` as follows:
+
+[source,java]
+----
+public interface CustomRegistryConfig extends StepRegistryConfig {
+
+	CustomRegistryConfig DEFAULT = k -> null;
+
+	@Override
+	default String prefix() {
+		return "custom";
+	}
+
+}
+----
+
+Second, define your own meter registry by extending `StepMeterRegistry` as follows:
+
+[source,java]
+----
+public class CustomMeterRegistry extends StepMeterRegistry {
+
+	public CustomMeterRegistry(CustomRegistryConfig config, Clock clock) {
+		super(config, clock);
+
+		start(new NamedThreadFactory("custom-metrics-publisher"));
+	}
+
+	@Override
+	protected void publish() {
+		getMeters().stream().forEach(meter -> System.out.println("Publishing " + meter.getId()));
+	}
+
+	@Override
+	protected TimeUnit getBaseTimeUnit() {
+		return TimeUnit.MILLISECONDS;
+	}
+
+}
+----
+
+Finally, create the meter registry configuration and the meter registry.
+If you are using Spring Boot, you can do as follows:
+
+[source,java]
+----
+@Configuration
+public class MetricsConfig {
+
+	@Bean
+	public CustomRegistryConfig customRegistryConfig() {
+		return CustomRegistryConfig.DEFAULT;
+	}
+
+	@Bean
+	public CustomMeterRegistry customMeterRegistry(CustomRegistryConfig customRegistryConfig, Clock clock) {
+		return new CustomMeterRegistry(customRegistryConfig, clock);
+	}
+
+}
+----