[ddsketch] add DDSketch.count() -> f64 to return number of points in the sketch

[brettlangdon]

What does this PR do?

Adds DDSketch.count() -> f64 function which returns the number of points currently in the sketch.

Motivation

For testing behavior like stats computation or datastreams, being able to assert that the bucketing into sketches works as intended would be helpful.

Additional Notes

How to test the change?

cargo test

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.29%. Comparing base (f380cb5) to head (ae7c810).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #532      +/-   ##
==========================================
+ Coverage   70.21%   70.29%   +0.07%     
==========================================
  Files         206      206              
  Lines       27815    27819       +4     
==========================================
+ Hits        19530    19554      +24     
+ Misses       8285     8265      -20     

Components	Coverage Δ
crashtracker	16.86% <ø> (ø)
datadog-alloc	98.73% <ø> (ø)
data-pipeline	51.15% <ø> (ø)
data-pipeline-ffi	0.00% <ø> (ø)
ddcommon	86.41% <ø> (ø)
ddcommon-ffi	75.31% <ø> (ø)
ddtelemetry	59.02% <ø> (ø)
ipc	84.13% <ø> (ø)
profiling	78.68% <ø> (ø)
profiling-ffi	58.26% <ø> (ø)
serverless	0.00% <ø> (ø)
sidecar	35.64% <ø> (ø)
sidecar-ffi	0.00% <ø> (ø)
spawn-worker	54.98% <ø> (ø)
trace-mini-agent	70.93% <ø> (ø)
trace-normalization	98.24% <ø> (ø)
trace-obfuscation	95.73% <ø> (ø)
trace-protobuf	77.16% <ø> (ø)
trace-utils	90.68% <ø> (+0.77%)	⬆️

[pr-commenter]

Benchmarks

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