[usm] system-probe, http statskeeper, add pool of ddsketch objects

[yuri-lipnesh]

What does this PR do?

Adds pool of DDSketch objects to StatKeeper struct.

Motivation

This is part of an effort to improve USM performance and reliability.
Profiling shows frequently allocated objects of 'DDSketch', using pool of these objects should decrease memory utilization.

Benchmarking on local VM shows shows allocation decrease 3.7 times:

pkg: github.com/DataDog/datadog-agent/pkg/network/protocols/http
BenchmarkHTTPStatkeeperWithPool-4   	     100	  10853091 ns/op	   86515 B/op	     651 allocs/op
BenchmarkHTTPStatkeeperNoPool-4     	     100	  11162432 ns/op	  338688 B/op	    2414 allocs/op

Describe how you validated your changes

Covered by unit tests.

Possible Drawbacks / Trade-offs

Additional Notes

[agent-platform-auto-pr]

Test changes on VM

Use this command from test-infra-definitions to manually test this PR changes on a VM:

inv create-vm --pipeline-id=50624672 --os-family=ubuntu

Note: This applies to commit d6b8218