[improve][broker] Add metrics related interfaces for BrokerInterceptor

[crossoverJie]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

When I want to monitor the relevant data in the BrokerInterceptor, currently the BrokerInterceptor does not provide relevant interfaces.

I think providing such extensions is necessary.

Solution

https://github.com/apache/pulsar/blob/e96b3398912163eb6e0528c10aed3507c95952fd/pulsar-broker/src/main/java/org/apache/pulsar/broker/intercept/BrokerInterceptor.java

Add a interface, like this:

default void addCustomizeMetrics(PrometheusMetricStreams streams, PulsarService pulsar){
}

pulsar/pulsar-broker/src/main/java/org/apache/pulsar/broker/stats/prometheus/PrometheusMetricsGenerator.java

Lines 185 to 187 in e96b339

	PrometheusMetricStreams metricStreams = new PrometheusMetricStreams();
	try {
	SimpleTextOutputStream stream = new SimpleTextOutputStream(buf);

Then callback the addCustomizeMetrics(stream, pulsar) method here.

This is just a draft.

Alternatives

No response

Anything else?

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!

[crossoverJie]

@codelipenghui Please take a look.

[asafm]

I think it's ok.
I presume you need high granularity metrics - topic level, which is why you want to use PrometheusMetricStreams, right?

This only emphasize the need to solve this properly via OpenTelemetry API and let plugins register their own metrics easily, as part of PIP-264: #20197
Once approved and we'll get to the plugin section, we'll add a proper method for registering metrics, which will consistent across all plugin interface and not bespoke as it is today.

[crossoverJie]

I read the PIP-264 and discussion threads, it sounds great, but it looks like it will take a lot of time investment.

Therefore, I think we can add this feature within the current context.

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