fix(serviceMonitor) set 30s as default interval

[dweber019]

What this PR does / why we need it:

The default interval should be 30s to support the rate of 1m of the official Kong Grafana dashboard.
Dashboard https://grafana.com/grafana/dashboards/7424-kong-official/

Which issue this PR fixes

N/A

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• PR is based off the current tip of the main branch.
• Changes are documented under the "Unreleased" header in CHANGELOG.md
• New or modified sections of values.yaml are documented in the README.md
• Commits follow the Kong commit message guidelines

[pangruoran]

Thanks for your contributions!

Actually, if we don't set a default value, it will use 30 seconds by default.

https://github.com/prometheus-operator/prometheus-operator/blob/79502954f995929c8b3511d533a2e34bb3ee40ca/pkg/apis/monitoring/v1/prometheus_types.go#L224-L228

Have you encountered any issues before making this change?

[dweber019]

Nice catch. We have indeed changed this in our Prometheus operator, so the issue was cased by our setup and not this chart or the Grafana dashboard.

Closing this PR. Thx