```ruby
require "spec_helper"

describe Rdkafka::Bindings do
  it "should load librdkafka" do
    expect(Rdkafka::Bindings.ffi_libraries.map(&:name).first).to include "librdkafka"
  end

  it "should successfully call librdkafka" do
    expect {
      Rdkafka::Bindings.rd_kafka_conf_new
    }.not_to raise_error
  end
end
```

Add specs for extension and log callback in bindings
user
```ruby
require "spec_helper"

describe Rdkafka::Bindings do
  it "should load librdkafka" do
    expect(Rdkafka::Bindings.ffi_libraries.map(&:name).first).to include "librdkafka"
  end

  describe ".lib_extension" do
    it "should know the lib extension for darwin" do
      expect(Gem::Platform.local).to receive(:os).and_return("darwin-aaa")
      expect(Rdkafka::Bindings.lib_extension).to eq "dylib"
    end

    it "should know the lib extension for linux" do
      expect(Gem::Platform.local).to receive(:os).and_return("linux")
      expect(Rdkafka::Bindings.lib_extension).to eq "so"
    end
  end

  it "should successfully call librdkafka" do
    expect {
      Rdkafka::Bindings.rd_kafka_conf_new
    }.not_to raise_error
  end

  describe "log callback" do
    let(:log) { StringIO.new }
    before do
      Rdkafka::Config.logger = Logger.new(log)
    end

    it "should log fatal messages" do
      Rdkafka::Bindings::LogCallback.call(nil, 0, nil, "log line")
      expect(log.string).to include "FATAL -- : rdkafka: log line"
    end

    it "should log error messages" do
      Rdkafka::Bindings::LogCallback.call(nil, 3, nil, "log line")
      expect(log.string).to include "ERROR -- : rdkafka: log line"
    end

    it "should log warning messages" do
      Rdkafka::Bindings::LogCallback.call(nil, 4, nil, "log line")
      expect(log.string).to include "WARN -- : rdkafka: log line"
    end

    it "should log info messages" do
      Rdkafka::Bindings::LogCallback.call(nil, 5, nil, "log line")
      expect(log.string).to include "INFO -- : rdkafka: log line"
    end

    it "should log debug messages" do
      Rdkafka::Bindings::LogCallback.call(nil, 7, nil, "log line")
      expect(log.string).to include "DEBUG -- : rdkafka: log line"
    end

    it "should log unknown messages" do
      Rdkafka::Bindings::LogCallback.call(nil, 100, nil, "log line")
      expect(log.string).to include "ANY -- : rdkafka: log line"
    end
  end
end
```
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