Set default RuleSampler rate_limit of 100 (#898)

* Set default RuleSampler rate_limit of 100

* use constant for default rate limit

lib/ddtrace/sampling/rule_sampler.rb

@@ -18,14 +18,16 @@ class RuleSampler
 
       attr_reader :rules, :rate_limiter, :default_sampler
 
+      DEFAULT_RATE_LIMIT = 100
+
       # @param rules [Array<Rule>] ordered list of rules to be applied to a span
-      # @param rate_limit [Float] number of traces per second, defaults to no rate limit
+      # @param rate_limit [Float] number of traces per second, defaults to +100+
       # @param rate_limiter [RateLimiter] limiter applied after rule matching
       # @param default_sample_rate [Float] fallback sample rate when no rules apply to a span,
       #   between +[0,1]+, defaults to +1+
       # @param default_sampler [Sample] fallback strategy when no rules apply to a span
       def initialize(rules = [],
-                     rate_limit: nil,
+                     rate_limit: DEFAULT_RATE_LIMIT,
                      rate_limiter: nil,
                      default_sample_rate: nil,
                      default_sampler: nil)

spec/ddtrace/sampling/rule_sampler_spec.rb

@@ -23,7 +23,7 @@
   context '#initialize' do
     subject(:rule_sampler) { described_class.new(rules) }
 
-    it { expect(subject.rate_limiter).to be_a(Datadog::Sampling::UnlimitedLimiter) }
+    it { expect(subject.rate_limiter).to be_a(Datadog::Sampling::TokenBucket) }
     it { expect(subject.default_sampler).to be_a(Datadog::AllSampler) }
 
     context 'with rate_limit' do
@@ -32,6 +32,12 @@
       it { expect(subject.rate_limiter).to be_a(Datadog::Sampling::TokenBucket) }
     end
 
+    context 'with nil rate_limit' do
+      subject(:rule_sampler) { described_class.new(rules, rate_limit: nil) }
+
+      it { expect(subject.rate_limiter).to be_a(Datadog::Sampling::UnlimitedLimiter) }
+    end
+
     context 'with default_sample_rate' do
       subject(:rule_sampler) { described_class.new(rules, default_sample_rate: 1.0) }