Indexing metrics_metadata.json file malformed

The file indexing/secondary/docs/metrics_metadata.json has several errors - missing or extraneous commas which make it not parse as valid JSON.

More generally, now that the production Server build attempts to process these files as JSON rather than just copying them (CBD-5626), errors like this will break the Server build. Therefore we should add steps to validate these files at CMake time so developers will notice problems.