[ENHANCEMENT] Capture metrics on which search query terms are used.

As the maintainer of webstatus.dev, I would like to understand which query terms are commonly used.

We could leverage OpenTelemetry to emit metrics on which terms are used. This could be instrumented in one of two places:
1. In the ANTLR visitor code when the nodes are being built.
2. In the spanner code when the filters are finally applied to the SQL query.

There are pros and cons to both.