A list of various Time-series Databases together with their estimated popularity can be found at http://db-engines.com/en/ranking/time+series+dbms

They list the implementation language for many systems, e.g. for Oracle: http://db-engines.com/en/system/Oracle Lines of code are typically not disclosed by the vendors; could be done for open-source databases, but then it's not obvious what really should be counted (eg. third party libraries/frameworks).

Yeah, it's not easy to measure, since there is so much use of third party libraries.