Merge code coverage output into single file for codecov.io

[evantorrie]

Fixes #300

This fixes an issue where code coverage was not being reported.

[codecov]

Codecov Report

Merging #514 (1cc6fd6) into master (26c2b18) will increase coverage by 77.61%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #514       +/-   ##
===========================================
+ Coverage        0   77.61%   +77.61%     
===========================================
  Files           0       54       +54     
  Lines           0     2586     +2586     
===========================================
+ Hits            0     2007     +2007     
- Misses          0      443      +443     
- Partials        0      136      +136     

Impacted Files	Coverage Δ
...ation/google.golang.org/grpc/otelgrpc/grpctrace.go	66.66% <0.00%> (ø)
...hub.com/emicklei/go-restful/otelrestful/restful.go	100.00% <0.00%> (ø)
...n/github.com/Shopify/sarama/otelsarama/producer.go	94.89% <0.00%> (ø)
...ation/github.com/astaxie/beego/otelbeego/config.go	100.00% <0.00%> (ø)
...strumentation/net/http/otelhttp/filters/filters.go	100.00% <0.00%> (ø)
detectors/gcp/cloud-run.go	93.93% <0.00%> (ø)
...github.com/Shopify/sarama/otelsarama/dispatcher.go	100.00% <0.00%> (ø)
...n/github.com/Shopify/sarama/otelsarama/consumer.go	100.00% <0.00%> (ø)
detectors/aws/ecs/ecs.go	54.54% <0.00%> (ø)
...mentation/net/http/otelhttp/filters/header_go14.go	100.00% <0.00%> (ø)
... and 44 more

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