fix: return nil for non-existant key in AwsSdk::MessageAttributeGetter

[yoheyk]

Since tracestate is optional, it can be omitted.

However, current implementation of AwsSdk::MessageAttributeGetter raises an exception here when it is omitted.

Same as TextMapGetter, it should return nil for non-existant key.

[xuan-cao-swi]

Looks good to me, just a little nit.
Would return unless carrier[key] introduce less code change?

[yoheyk]

it is less code change but since it is doing carrier[key] multiple times, it seems to be a little slower:

def get_with_variable(carrier, key)
  message_attribute = carrier[key]
  message_attribute[:string_value] if message_attribute && message_attribute[:data_type] == 'String'
end

def get_with_return(carrier, key)
  return unless carrier[key]

  carrier[key][:string_value] if carrier[key][:data_type] == 'String'
end

carrier = { 'traceparent' => { data_type: 'String', string_value: 'tp' } }

Benchmark.ips do |x|
  x.report("get_with_variable") { get_with_variable(carrier, 'traceparent') }
  x.report("get_with_return") { get_with_return(carrier, 'traceparent') }

  x.compare!
end

Calculating -------------------------------------
   get_with_variable      4.959M (± 7.8%) i/s -     24.754M in   5.053304s
     get_with_return      4.337M (± 2.0%) i/s -     21.870M in   5.045064s

Comparison:
   get_with_variable:  4959117.6 i/s
     get_with_return:  4336852.7 i/s - 1.14x  slower

but either way is fine. let me know if I should change the code

[xuan-cao-swi]

Thanks for putting up the comparison. I am ok with better performance.

[arielvalentin]

@yoheyk Thank you for your contribution.

We are currently looking for help maintaining AWS related instrumentation.

Would you be interested?

[yoheyk]

Thanks for merging and releasing so quickly!

We are currently looking for help maintaining AWS related instrumentation.

Would you be interested?

@arielvalentin yeah, I think I can help to some extent.