How does StepMeterRegistry publish method execute in spring-boot? - spring-boot-actuator

Question

package io.micrometer.core.instrument;
public interface Meter {
///...
default <T> T match(Function<Gauge, T> visitGauge, Function<Counter, T> visitCounter, Function<Timer, T> visitTimer,
Function<DistributionSummary, T> visitSummary, Function<LongTaskTimer, T> visitLongTaskTimer,
Function<TimeGauge, T> visitTimeGauge, Function<FunctionCounter, T> visitFunctionCounter,
Function<FunctionTimer, T> visitFunctionTimer, Function<Meter, T> visitMeter) {
if (this instanceof TimeGauge) {
return visitTimeGauge.apply((TimeGauge) this);
}
else if (this instanceof Gauge) {
return visitGauge.apply((Gauge) this);
}
else if (this instanceof Counter) {
return visitCounter.apply((Counter) this);
}
else if (this instanceof Timer) {
return visitTimer.apply((Timer) this);
}
else if (this instanceof DistributionSummary) {
return visitSummary.apply((DistributionSummary) this);
}
else if (this instanceof LongTaskTimer) {
return visitLongTaskTimer.apply((LongTaskTimer) this);
}
else if (this instanceof FunctionCounter) {
return visitFunctionCounter.apply((FunctionCounter) this);
}
else if (this instanceof FunctionTimer) {
return visitFunctionTimer.apply((FunctionTimer) this);
}
else {
return visitMeter.apply(this);
}
}
//...
}
public class XyzMeterRegistry extends StepMeterRegistry {
//...
#Override
protected void publish() {
for (List<Meter> batch : MeterPartition.partition(this, config.batchSize())) {
try {
// #formatter:off
httpClient.post(config.uri())
.withBasicAuthentication(config.userName(), config.password())
.withJsonContent(
batch.stream().flatMap(m -> m.match(
this::writeGauge,
this::writeCounter,
this::writeTimer,
this::writeSummary,
this::writeLongTaskTimer,
this::writeTimeGauge,
this::writeFunctionCounter,
this::writeFunctionTimer,
this::writeCustomMetric)
).collect(Collectors.joining(",", "[", "]"))
)
// #formatter:on
.send()
.onSuccess(response -> logger.debug("successfully sent {} metrics to kairos.", batch.size()))
.onError(response -> logger.error("failed to send metrics to kairos: {}", response.body()));
}
catch (Throwable t) {
logger.warn("failed to send metrics to kairos", t);
}
}
}
Stream<String> writeGauge(Gauge gauge) {
double value = gauge.value();
if (Double.isFinite(value)) {
return Stream.of(writeMetric(gauge.getId(), config().clock().wallTime(), value));
}
return Stream.empty();
}
Stream<String> writeCounter(Counter counter) {
return Stream.of(writeMetric(counter.getId(), config().clock().wallTime(), counter.count()));
}
Stream<String> writeTimer(Timer timer) {
long wallTime = config().clock().wallTime();
return Stream.of(writeMetric(idWithSuffix(timer.getId(), "count"), wallTime, timer.count()),
writeMetric(idWithSuffix(timer.getId(), "max"), wallTime, timer.max(getBaseTimeUnit())),
writeMetric(idWithSuffix(timer.getId(), "avg"), wallTime, timer.mean(getBaseTimeUnit())),
writeMetric(idWithSuffix(timer.getId(), "sum"), wallTime, timer.totalTime(getBaseTimeUnit())));
}
Stream<String> writeFunctionCounter(FunctionCounter counter) {
double count = counter.count();
if (Double.isFinite(count)) {
return Stream.of(writeMetric(counter.getId(), config().clock().wallTime(), count));
}
return Stream.empty();
}
//...etc.,
}
How the above match( ) is called from MeterRegistryComponent's publish method?
//How does here m.match(//....) is taking parameters from MeterRegistryComponent's publish method?**
//And what is the role of flatMap here?**
I just want to write custom meterregistry in which need to publish metrics using publish method