fix: report total threads as gauge (#2481)

[ousamabenyounes]

Summary

Fix #2481

• Register frankenphp_total_threads as a Prometheus gauge.
• Report the current configured thread count with Set instead of accumulating with Add.
• Update metric test expectations for the Caddy integration tests.

Test verification (RED -> GREEN)

RED, on main with only the new regression test added:

--- FAIL: TestPrometheusMetrics_TotalThreadsReportsCurrentValue (0.00s)
    metrics_test.go:40:
        Error:       Received unexpected error:
                     # HELP frankenphp_total_threads Total number of PHP threads
                    -# TYPE frankenphp_total_threads counter
                    -frankenphp_total_threads 5
                    +# TYPE frankenphp_total_threads gauge
                    +frankenphp_total_threads 2
FAIL

GREEN, after the fix:

ok  	command-line-arguments	0.009s

Additional targeted metrics run:

ok  	command-line-arguments	0.008s

Full local suite

Local environment notes:

• The host does not have Go installed, so validation was run in Docker.
• go test ./... in a plain Go container failed before tests because native PHP/watcher headers were absent.
• go test -tags nowatcher ./... with php-dev reached native linking but failed because the minimal container lacked PHP link libraries.
• After adding PHP link libraries, the focused root-package command passed:

ok  	github.com/dunglas/frankenphp	0.032s

• The complete root-package run with the same native dependencies was killed by the container with exit code 137 before producing test results.

Files changed

File	Change
metrics.go	Change totalThreads from Counter to Gauge and use Set.
metrics_test.go	Add regression coverage for type and current-value semantics.
caddy/caddy_test.go	Update expected metric type to gauge.

[alexandre-daubois]

Thank you @ousamabenyounes