ensure tags are preserved for fetchExists(Table, Condition)

[HeeChanN]

Override MetricsDSLContext#fetchExists(Table, Condition) to delegate to
super.fetchExists(DSL.selectOne().from(table).where(condition)), forcing the
standard SELECT path so that JooqExecuteListener receives the tags and metrics
are reported with the provided tags.

Tests:

• fetchExistsIsTimedWithProvidedTags: verifies that calling
  dsl.tag("name","checkAuthorExists").fetchExists(...) records "jooq.query"
  with tags name=checkAuthorExists and type=read, and timer count = 1 on H2.

related #6583

[shakuzen]

Thank you for the pull request with a fix and a test for the issue. I'll rebase it on the 1.14.x branch so we can easily include it in upcoming maintenance releases.