feat(metrics): log warning on empty metrics

[dreamorosi]

Description of your changes

When using the logMetrics decorator or middleware users can pass a throwOnEmptyMetrics parameter to set the Metrics utility to throw an error when metrics are being flushed but no metric is actually present. In those cases when throwOnEmptyMetrics is set to false and no metric is added however the utility "fails silently" giving the user no way of detecting potentially missing metrics.

This PR introduces a new warning log that reads:

No application metrics to publish. The cold-start metric may be published if enabled. If application metrics should never be empty, consider using 'throwOnEmptyMetrics',

This warning will be logged whenever the user tries to flush metrics but no metric was stored and throwOnEmptyMetrics is set to false or not set.

Once merged this PR closes #1291.

Related issues, RFCs

Issue number: #1291

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published
• The PR title follows the conventional commit semantics

Breaking change checklist

Is it a breaking change?: NO

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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[revmischa]

It would be nice if there was a way to disable this warning.