Add metrics collection for expression evaluation

Define, review and implement metrics collection for expression evaluation.

Metrics should include at the very least performance and failure rate.

No files or tests are named. Start by locating the expression-evaluation entry point and reviewing how it is exercised, then determine the project's existing metrics conventions; the work is done when performance and failure-rate metrics are defined, reviewed, implemented, and covered by appropriate verification.

Written by the indexing model from the issue text.