ISSUE-18243: Add observability metrics for CommandPartitionedTopicMetadata requests

[sijie]

Original Issue: apache#18243

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• I searched in the issues and found nothing similar.

Motivation

Currently, there's no way to track CommandPartitionedTopicMetadata requests. There's no metrics or logs that indicate that a broker is handling CommandPartitionedTopicMetadata requests.

Misconfigured clients might flood brokers with CommandPartitionedTopicMetadata requests and cause high CPU consumption.

One example of this is misconfiguration of splunk-otel-collector's Pulsar exporter. The example config configures pulsar-client-go's PartitionsAutoDiscoveryInterval setting to 1 nanosecond. I have sent a PR to fix the example config with signalfx/splunk-otel-collector#2185 . This example shows that it's easy to mix the units and misconfigure a Pulsar client.

Solution

Add observability metrics for CommandPartitionedTopicMetadata requests, similar to what there is for lookup requests added by apache#8272.

Alternatives

No response

Anything else?

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!

[tjiuming]

apache#18281