pkg/config: Allow configuring to keep only certain sample types

[brancz]

If a config specifies a keep_sample_types allow-list in then scrape config then we'll parse the scraped pprof profile and keep only those samples within a profile that match one of the listed sample types. This is especially useful for cases where users only want to ingest heap profiling data, for which there is no way in the pprof endpoints in Go to ignore other data at collection time.

[alwaysmeticulous]

✅ Meticulous spotted zero visual differences across 352 screens tested: view results.

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