Log the source of sampling strategies

[yurishkuro]

Help with #2197

$ go run ./cmd/all-in-one --sampling.strategies-file cmd/all-in-one/sampling_strategies.json
...
{"level":"info","ts":1627395862.803787,"caller":"static/strategy_store.go:138","msg":"Loading sampling strategies","filename":"cmd/all-in-one/sampling_strategies.json"}
...

[codecov]

Codecov Report

Merging #3166 (31a2f9e) into master (779027c) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3166      +/-   ##
==========================================
- Coverage   95.92%   95.91%   -0.01%     
==========================================
  Files         239      239              
  Lines       14647    14649       +2     
==========================================
+ Hits        14050    14051       +1     
- Misses        520      521       +1     
  Partials       77       77              

Impacted Files	Coverage Δ
...in/sampling/strategystore/static/strategy_store.go	97.26% <100.00%> (+0.02%)	⬆️
cmd/query/app/server.go	94.11% <0.00%> (-1.48%)	⬇️
plugin/storage/integration/integration.go	78.88% <0.00%> (-0.40%)	⬇️
cmd/query/app/static_handler.go	97.60% <0.00%> (+1.79%)	⬆️

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