C++: Add expressions with type data to cpp/extraction-information

[paldepind]

Follow up from #21512. Extends the query to also include data for expressions in the source with types. Like before the QL is more or less copy pasted from the similar queries for Rust/Java/C#.

Here's a comparison table for nmap similar to the one in the previous PR. The first row is the existing data, and the three below are from cpp/telemetry/extraction-information.

	traced	BMN	BMN + deps inst
Expressions with a known type (existing data)	433,251	729,293	850,145
Percentage of expressions with known type	99.999	98.985	98.976
Number of expressions with known type	285,612	585,821	500,554
Number of expressions with unknown type	2	6,009	5,178

Again, the new numbers show more clearly the actual changes from BMN and dependency installation. Though at least for this project the call target data seemed more revealing. Two observations:

• The increase in the number of expressions with a known type is relatively much greater than the increase in the number of calls with a target.
• The percentage of expressions with a known type stays very high in both BMN cases. This seems to suggest that the reduction in expressions with a known type for dependency installation is because expressions are completely lost (and not just extracted without a type).

Perhaps others have some additional insight, but to me it seems that the resolved calls metric is more significant.

Here's the data shown in DCA.

[Copilot]

Pull request overview

Extends the C/C++ cpp/telemetry/extraction-information metric query to also report statistics about expression type availability for expressions originating from source files, complementing the existing “resolved call target” quality metrics.

Changes:

• Added expression type quality metrics (known vs unknown/erroneous type) to the extraction-information telemetry output.
• Introduced shared “relevant source file” filtering to ensure metrics are restricted to source files with a relative path.

Reviewed changes

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

File	Description
cpp/ql/src/Telemetry/ExtractorInformation.ql	Adds expression-type stats keys/values to the emitted telemetry metrics alongside call-target stats.
cpp/ql/src/Telemetry/DatabaseQuality.qll	Defines the expression-type stats computation and refactors source-file scoping via a RelevantFile helper.

[Copilot]

The doc comment for RelevantFile is a bit vague given the actual predicate (source file + has relative path). Consider clarifying that this is specifically a source file included in the database-quality metrics (and why exists(getRelativePath()) is part of the criteria).

Suggested change

	/** A file that is included in the quality statistics. */
	/**Asourcefile that is included in the database-quality statistics, that is,
	*afilecomingfrom the source code (`fromSource()`)and having a relative
	*pathinthedatabase(`exists(getRelativePath())`),sothatonly files
	*properlymappedintothedatabasefiletreeare counted. */

[geoffw0]

Otherwise LGTM.

[geoffw0]

The purpose of this predicate isn't clear to me, and it's unused.

[paldepind]

Thanks, I used it during testing and forgot to remove it. Fixed in 8cb5380.