perf(dogstatsd): optimize flush_metrics to skip processing when empty

[litianningdatadog]

perf(dogstatsd): optimize flush_metrics to skip processing when empty

https://datadoghq.atlassian.net/browse/SVLS-7845

• Skip expensive operations in flush_metrics() when both series and distributions are empty. Reduces unnecessary logs if no meaningful flush occurs
• Adds unit tests proving the optimization works correctly
• Fixed clippy warning fix.

Test:

	Before	After
Log