feat: add estimate_pending_compaction_bytes metric to analyze fragmentation rate

[yaoyinnan]

Add estimate_pending_compaction_bytes metric to analyze fragmentation rate.

Serial Number	Metric	Meaning
40	rocksdb.estimate-pending-compaction-bytes	Estimated total number of bytes that compression needs to rewrite to bring all levels down below the target size. Has no effect on compression other than level-based compression.

Fixes: #1665

[cr-gpt]

Based on the code patch provided, here are some observations and suggestions:

Observations:

• The code is adding a new line to write the "estimate_pending_compaction_bytes" property to the info stream.
• It appears that this code is part of a larger function called GetRocksDBInfo.
• No other specific details about the overall function or the context in which this code is used are provided.

Suggestions:

1. Confirm Dependency: Ensure that you have verified that the rocksdb::DB::Properties::kEstimatePendingCompactionBytes property is available and works as expected in your version of RocksDB.

2. Error Handling: It's important to handle any potential errors that may occur during the process of retrieving and writing the properties. Make sure appropriate error handling mechanisms (e.g., try-catch) are implemented.

3. Code Documentation: Consider adding comments to describe the purpose of this function and the significance of each property being written to the info stream.

4. Review Other Properties: Since this code patch only includes the addition of the "estimate_pending_compaction_bytes" property, it's worth considering if there are any other relevant properties that should be included in the info stream.

5. Unit Testing: Test the functionality of this code patch thoroughly to ensure it produces the desired results and doesn't introduce any unexpected issues. Verify that the info stream contains all the expected properties and their corresponding values.

Remember that without additional context and a complete understanding of your codebase and requirements, it's difficult to provide a comprehensive review.

[cr-gpt]

The code patch you provided adds a new metric configuration for "estimate_pending_compaction_bytes" to the existing collectRocksDBMetrics map. Here's a brief code review:

1. Syntax and Formatting: The code appears to be formatted correctly, with indentation and line breaks.

2. Regular Expression: The regular expression used in the reg field of the regexParser struct seems fine. However, it's always good to thoroughly test a regular expression to ensure it matches the intended patterns.

3. MetaData Configuration: The MetricMeta struct defines metadata for the new metric. It includes the metric name, help description, type (presumably "gauge"), labels, and value name. As long as these fields are accurate and appropriate for your use case, the configuration seems fine.

Overall, from the given code patch, there don't appear to be any obvious bugs or risks. However, it's essential to thoroughly test the code to ensure proper functionality. Additionally, it's challenging to provide further improvement suggestions without more context about the codebase and its requirements.