Change the pulsar_client_sending_buffers_count metric to client level

[BewareMyPower]

#1394 introduces the pulsar_client_sending_buffers_count metric to track how many buffers are allocated for send purpose and not put back to the pool. However, unlike other metrics, this metric is not client level, so it cannot be attached with CustomMetricsLabels in client options.

When a send buffer was not put back to the pool, it means the Release method is not called due to some reason. Changing this metric to client level could help locate which client has triggered this bug in an application that has many client instances from different businesses.

Here is an example metric when I configured CustomMetricsLabels: map[string]string{"key": "value"} after this change

pulsar_client_sending_buffers_count{client="go",key="value"} 1

[crossoverJie]

Can you explain the motivation for this change?

[BewareMyPower]

@crossoverJie I have updated the PR description, PTAL again

[Copilot]

Pull Request Overview

This PR moves the pulsar_client_sending_buffers_count metric from a global level to client level, enabling it to be tagged with custom metrics labels. This change allows better tracking of buffer allocation issues across multiple client instances in applications.

Key changes include:

• Refactored metric creation to use client-specific labels instead of global constants
• Updated buffer allocation tracking to use client-level metrics
• Modified batch builders and producers to accept metrics as a parameter

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pulsar/internal/metrics.go	Moved SendingBuffersCount from global variable to client-level metric with custom labels support
pulsar/internal/buffer.go	Added release callback mechanism to track buffer lifecycle and removed global metric tracking
pulsar/internal/batch_builder.go	Updated to accept metrics parameter and track buffer allocation at client level
pulsar/internal/key_based_batch_builder.go	Updated function signatures to pass metrics parameter through batch container creation
pulsar/internal/key_based_batch_builder_test.go	Added metrics parameter to test setup
pulsar/producer_partition.go	Updated to pass client metrics to batch builder and track buffer allocation in single send

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[crossoverJie]

LGTM