Fix support for Prometheus Remote Read protocol

[vitlibar]

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Changelog entry (a user-readable short description of the changes that goes into CHANGELOG.md):

Fix support for Prometheus Remote Read protocol

[clickhouse-gh]

Workflow [PR], commit [33b3990]

Summary: ❌

job_name	test_name	status	info	comment
Integration tests (amd_asan, db disk, old analyzer, 6/6)		failure
	test_kafka_bad_messages/test_mv_target_missing.py::test_missing_mv_transitive_target[generate_old_create_table_query]	FAIL	cidb, issue
Integration tests (arm_binary, distributed plan, 4/4)		failure
	test_storage_s3_queue/test_0.py::test_move_after_processing[another_bucket-S3Queue]	FAIL	cidb
Integration tests (amd_tsan, 3/6)		error
	test_backup_restore_on_cluster/test_disallow_concurrency.py::test_concurrent_backups_on_same_node	FAIL	cidb, issue
	test_backup_restore_on_cluster/test_disallow_concurrency.py::test_concurrent_backups_on_different_nodes	FAIL	cidb, issue
	test_backup_restore_on_cluster/test_disallow_concurrency.py::test_concurrent_restores_on_same_node	FAIL	cidb, issue
	test_backup_restore_on_cluster/test_disallow_concurrency.py::test_concurrent_restores_on_different_node	FAIL	cidb, issue

[Copilot]

Pull request overview

This PR fixes support for the Prometheus Remote Read protocol by addressing a buffer management issue in protobuf serialization and enhancing test coverage with a large dataset test.

Key changes:

• Added nextIfAtEnd() call in ProtobufZeroCopyOutputStreamFromWriteBuffer::Next() to ensure buffer availability before writing
• Introduced comprehensive test for Remote Read protocol with 75,000 data points
• Refactored test setup to populate data once during cluster initialization rather than per-test

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/IO/Protobuf/ProtobufZeroCopyOutputStreamFromWriteBuffer.cpp	Fixed buffer handling by ensuring next buffer chunk is available before writing
tests/integration/test_prometheus_protocols/test_write_read.py	Added large dataset test, refactored data population to fixture, and renamed wait function for clarity

[vitlibar]

CI failures are unrelated:

• test_backup_restore_on_cluster/test_disallow_concurrency.py - already fixed
• test_storage_s3_queue/test_0.py::test_move_after_processing[another_bucket-S3Queue] - see Flaky test: test_storage_s3_queue/test_0.py::test_move_after_processing[another_bucket-S3Queue] #93919