Support sampling startegies file flag in OTEL collector

[pavolloffay]

Resolves #2154

This patch showcases how upstream OTEL components can be extended to support additional means of configuration. In this case I have added sampling.strategies-file flag to Jaeger receiver. The configuration in OTEL config has higher precedence over the configuration in Viper (like we do for other jaeger flags).

• if the flag sampling.strategies-file is used and the sampling strategies in the OTEL config is missing it would enable it. e.g.:

go run main.go --config=/home/ploffay/tmp/otel-config.yml  --sampling.strategies-file=/home/ploffay/tmp/sampling.json
receivers:
  jaeger:
    protocols:
      grpc:  

Now let's talk about how this approach could work for other flags. Flags that don't have a default value can be supported easily. However flags with a default value e.g. --collector.http-server.host-port=:14268 are problematic. The problem is that when the default config is created we don't know whether this endpoint should be enabled or disabled as there is always a default value. When the collector is started without a configuration file it's not a problem because we enable all Jaeger endpoints. However when the collector is started with a configuration file the default value in the flag always enables the endpoint - which might be problematic if users want to disable the endpoint.

[codecov]

Codecov Report

Merging #2195 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2195      +/-   ##
==========================================
+ Coverage   96.16%   96.18%   +0.01%     
==========================================
  Files         219      219              
  Lines       10632    10632              
==========================================
+ Hits        10224    10226       +2     
+ Misses        352      351       -1     
+ Partials       56       55       -1     

Impacted Files	Coverage Δ
plugin/sampling/strategystore/static/options.go	100.00% <100.00%> (ø)
cmd/query/app/static_handler.go	88.59% <0.00%> (+1.75%)	⬆️

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[objectiser]

However when the collector is started with a configuration file the default value in the flag always enables the endpoint - which might be problematic if users want to disable the endpoint.

I'm assuming we are only talking about the custom Jaeger build of the OTel Collector? If so, can't we remove the default values and have them moved to the default OTel Collector config?

So then, the flags are only override values.

[objectiser]

What is the reason for adding this?

[pavolloffay]

This can print out an error in some situations. getOTELConfigFile is a hack that parses flags before actual parsing by cobra command so it's fine to ignore it. If there is an error it is recognized once the command runs.

[pavolloffay]

PR rebased and updated