[jOOQ] Fix double instrumentation in MetricsDSLContext.fetchValue(SelectField) (#6659)

[bhawnapannu2701]

Problem
jOOQ 3.20 introduced a new DefaultDSLContext#fetchValue(SelectField) overload
that internally delegates to select(field).fetchValue(). With builder-level
instrumentation in MetricsDSLContext, that delegation leads to double
instrumentation (two jooq.query timers) and tag loss.

Solution
Override MetricsDSLContext.fetchValue(SelectField):

• For TableField: build the select via this context (select(field).from(table).fetchOne().value1()) so instrumentation triggers once.
• For non-table fields: delegate to super.fetchValue(field) to avoid an extra select(...) hop.

Verification
New JUnit 5 test MetricsDSLContextFetchValueTest executes
jooq.tag("name","fetchValue").fetchValue(DSL.inline(42)) against H2.
Before this change two samples were recorded; now exactly one sample is recorded.

Fixes #6659.