fix: reject non-positive ingest sample rates

[immanuwell]

tiny footgun, but real.

/ingest accepts sampleRate from the query string. Before this patch, sampleRate=-1 was parsed as an int and then cast to uint32, so it wrapped to 4294967295. For CPU and wall profiles that can drive the computed period down to 0, which is pretty cursed.

fix:

• treat sampleRate <= 0 as invalid
• fall back to the documented default 100
• add tests for -1, 0, abc, and a valid positive value

repro:

1. hit /ingest?...&sampleRate=-1
2. before this patch, the handler stored 4294967295 as the sample rate
3. after this patch, it falls back to 100

verified with go test ./pkg/ingester/pyroscope -count=1

[simonswine]

This make perfect sense. Thanks for adding it

LGTM

[korniltsev-grafanista]

Were there any problems with sample rate 0? I noticed a ton of errors in my logs #5276