Allow RateSampler with a sampling rate of zero

[marcotc]

2.0 Upgrade Guide notes

The custom sampler class Datadog::Tracing::Sampling::RateSampler now accepts a sample_rate of zero. This will drop all spans. Before 2.0, the RateSampler would fall back to a sample rate of 1.0 when provided with a sample_rate of zero.

Motivation:

This PR creates consistency across all sampling configurations: they all allowed a zero sampling rate, only RateSampler was the exception.

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (2.0@08a0d56). Click here to learn what that means.

Additional details and impacted files

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