README.md in opentelemetry-processor-baggage-0.1.0 vs README.md in opentelemetry-processor-baggage-0.2.0

@@ -1,10 +1,10 @@
 # Baggage Span Processor
 
 This is an OpenTelemetry [span processor](https://opentelemetry.io/docs/specs/otel/trace/sdk/#span-processor) that reads key/values stored in [Baggage](https://opentelemetry.io/docs/specs/otel/baggage/api/) in the starting span's parent context and adds them as attributes to the span.
 
-Keys and values added to Baggage will appear on all subsequent child spans for a trace within this service *and* will be propagated to external services via propagation headers.
+Keys and values added to Baggage will appear on all subsequent child spans, not the current active span, for a trace within this service *and* will be propagated to external services via propagation headers.
 If the external services also have a Baggage span processor, the keys and values will appear in those child spans as well.
 
 ⚠️ Waning ⚠️
 To repeat: a consequence of adding data to Baggage is that the keys and values will appear in all outgoing HTTP headers from the application.
 Do not put sensitive information in Baggage.
@@ -29,21 +29,24 @@
 
 ```ruby
 gem 'opentelemetry-processor-baggage'
 ```
 
-Then add the processor to an SDK's configuration:
+Then configure the span processor to copy all baggage entries:
 
 ```ruby
 require 'rubygems'
 require 'bundler/setup'
 
 Bundler.require
 
 OpenTelemetry::SDK.configure do |c|
-  # Add the BaggageSpanProcessor to the collection of span processors
-  c.add_span_processor(OpenTelemetry::Processor::Baggage::BaggageSpanProcessor.new)
+  # Add the BaggageSpanProcessor to the collection of span processors and
+  # copy all baggage entries
+  c.add_span_processor(OpenTelemetry::Processor::Baggage::BaggageSpanProcessor.new(
+    OpenTelemetry::Processor::Baggage::ALLOW_ALL_BAGGAGE_KEYS
+  ))
 
   # Because the span processor list is no longer empty, the SDK will not use the
   # values in OTEL_TRACES_EXPORTER to instantiate exporters.
   # You'll need to declare your own here in the configure block.
   #
@@ -53,9 +56,30 @@
     OpenTelemetry::SDK::Trace::Export::BatchSpanProcessor.new(
       OpenTelemetry::Exporter::OTLP::Exporter.new()
     )
   )
 end
+```
+
+Alternatively, you can provide a custom baggage key predicate to select which baggage keys you want to copy.
+
+For example, to only copy baggage entries that start with `myapp.`:
+
+```ruby
+OUR_BAGGAGE_KEY_PREFIX = 'myapp.'.freeze
+OpenTelemetry::Processor::Baggage::BaggageSpanProcessor.new(
+  # a constant here improves performance
+  ->(baggage_key) { baggage_key.start_with?(OUR_BAGGAGE_KEY_PREFIX) }
+)
+```
+
+For example, to only copy baggage entries that match `myapp.`, `myapp1.` and `myapp42.`:
+
+```ruby
+OUR_BAGGAGE_KEY_MATCHER = /\Amyapp\d*\./
+OpenTelemetry::Processor::Baggage::BaggageSpanProcessor.new(
+  ->(baggage_key) { OUR_BAGGAGE_KEY_MATCHER.match?(baggage_key) }
+)
 ```
 
 ## How can I get involved?
 
 The `opentelemetry-processor-baggage` gem source is [on github][repo-github], along with related gems including `opentelemetry-api` and `opentelemetry-sdk`.