Use alternative context implementation

We should still do some benchmarking, but I think this is going to be a
better implementation, so I'm preemptively using it for the purpose of
the initial PR.

api/lib/opentelemetry/context.rb

@@ -10,7 +10,6 @@ module OpenTelemetry
   # Manages context on a per-thread basis
   class Context
     KEY = :__opentelemetry_context__
-    EMPTY_ENTRIES = {}.freeze
 
     class << self
       # Returns current context, which is never nil
@@ -80,21 +79,26 @@ def clear
       end
 
       def empty
-        new(nil, EMPTY_ENTRIES)
+        new(nil, nil, nil)
       end
     end
 
-    def initialize(parent = nil, entries = {})
+    def initialize(parent, key, value)
       @parent = parent
-      @entries = entries.freeze
+      @key = key
+      @value = value
     end
 
     # Returns the corresponding value (or nil) for key
     #
     # @param [String] key The lookup key
     # @return [Object]
     def value(key)
-      @entries[key]
+      if key == @key
+        @value
+      elsif @parent
+        @parent.value(key)
+      end
     end
 
     alias [] value
@@ -105,9 +109,7 @@ def value(key)
     # @param [Object] value Object to be stored under key
     # @return [Context]
     def set_value(key, value)
-      new_entries = @entries.dup
-      new_entries[key] = value
-      Context.new(self, new_entries)
+      Context.new(self, key, value)
     end
 
     # Returns a new Context with the current context's entries merged with the
@@ -118,7 +120,7 @@ def set_value(key, value)
     # @param [Object] value Object to be stored under key
     # @return [Context]
     def set_values(values) # rubocop:disable Naming/AccessorMethodName:
-      Context.new(self, @entries.merge(values))
+      values.inject(self) { |parent, (k, v)| Context.new(parent, k, v) }
     end
 
     # Makes the this context the currently active context and returns the

api/test/opentelemetry/context_test.rb

@@ -15,6 +15,8 @@
     Context.clear
   end
 
+  let(:new_context) { Context.new(nil, 'foo', 'bar') }
+
   describe '.current' do
     it 'defaults to the root context' do
       _(Context.current).must_equal(Context::ROOT)
@@ -23,7 +25,7 @@
 
   describe '.with_current' do
     it 'handles nested contexts' do
-      c1 = Context.new(nil, 'foo' => 'bar')
+      c1 = new_context
       Context.with_current(c1) do
         _(Context.current).must_equal(c1)
         c2 = Context.current.set_value('bar', 'baz')
@@ -35,7 +37,7 @@
     end
 
     it 'resets context when an exception is raised' do
-      c1 = Context.new(nil, 'foo' => 'bar')
+      c1 = new_context
       Context.current = c1
 
       _(proc do
@@ -68,7 +70,7 @@
 
   describe '#value' do
     it 'returns corresponding value for key' do
-      ctx = Context.new(nil, 'foo' => 'bar')
+      ctx = new_context
       _(ctx.value('foo')).must_equal('bar')
     end
   end
@@ -93,15 +95,15 @@
 
   describe '#set_values' do
     it 'assigns multiple values' do
-      ctx = Context.new(nil, 'foo' => 'bar')
+      ctx = new_context
       ctx2 = ctx.set_values('bar' => 'baz', 'baz' => 'quux')
       _(ctx2.value('foo')).must_equal('bar')
       _(ctx2.value('bar')).must_equal('baz')
       _(ctx2.value('baz')).must_equal('quux')
     end
 
     it 'merges new values' do
-      ctx = Context.new(nil, 'foo' => 'bar')
+      ctx = new_context
       ctx2 = ctx.set_values('foo' => 'foobar', 'bar' => 'baz')
       _(ctx2.value('foo')).must_equal('foobar')
       _(ctx2.value('bar')).must_equal('baz')
@@ -152,7 +154,7 @@
     end
 
     it 'restores root for ctx without parent' do
-      ctx = Context.new(nil, 'foo' => 'bar')
+      ctx = new_context
       ctx.detach
       _(Context.current).must_equal(Context::ROOT)
     end
@@ -184,7 +186,7 @@
 
   describe 'threading' do
     it 'unwinds the stack on each thread' do
-      ctx = Context.new(nil, 'foo' => 'bar')
+      ctx = new_context
       t1_ctx_before = Context.current
       Context.with_current(ctx) do
         Thread.new do
@@ -204,7 +206,7 @@
     end
 
     it 'scopes changes to the current thread' do
-      ctx = Context.new(nil, 'foo' => 'bar')
+      ctx = new_context
       Context.with_current(ctx) do
         Thread.new do
           Context.with_current(ctx) do