Changed sampling type env var and updated collector help text

[joe-elliott]

Which problem is this PR solving?

Short description of the changes

As suggested here: jaegertracing/documentation#526

This PR renamed SAMPLING_TYPE env var to SAMPLING_CONFIG_TYPE. Additionally it extends the collector help text to include information about this new env var.

New text:

$ go run ./cmd/collector env
2021/10/04 10:00:45 maxprocs: Leaving GOMAXPROCS=8: CPU quota undefined

All command line options can be provided via environment variables by converting
their names to upper case and replacing punctuation with underscores. For example:

command line option                 environment variable
------------------------------------------------------------------
--cassandra.connections-per-host    CASSANDRA_CONNECTIONS_PER_HOST
--metrics-backend                   METRICS_BACKEND

The following configuration options are only available via environment variables:

SPAN_STORAGE_TYPE string         The type of backend [cassandra, opensearch, elasticsearch, memory, kafka, badger, grpc-plugin] used for trace storage. Multiple backends can be specified as comma-separated list, e.g. "cassandra,elasticsearch" (currently only for writing spans). Note that "kafka" is only valid in jaeger-collector; it is not a replacement for a proper storage backend, and only used as a buffer for spans when Jaeger is deployed in the collector+ingester configuration.  (default "cassandra")

DEPENDENCY_STORAGE_TYPE string   The type of backend used for service dependencies storage. (default "${SPAN_STORAGE_TYPE}")

SAMPLING_CONFIG_TYPE string      The method [file, adaptive] used for determining the sampling rates served to clients configured with remote sampling enabled. "file" uses a periodically reloaded file and "adaptive" dynamically adjusts sampling rates based on current traffic.  (default "file")

[codecov]

Codecov Report

Merging #3302 (a7b20df) into master (2b7fb73) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3302      +/-   ##
==========================================
- Coverage   96.01%   95.98%   -0.04%     
==========================================
  Files         259      259              
  Lines       15413    15434      +21     
==========================================
+ Hits        14799    14814      +15     
- Misses        522      526       +4     
- Partials       92       94       +2     

Impacted Files	Coverage Δ
cmd/env/command.go	100.00% <100.00%> (ø)
plugin/sampling/strategystore/factory.go	100.00% <100.00%> (ø)
plugin/sampling/strategystore/factory_config.go	100.00% <100.00%> (ø)
cmd/query/app/static_handler.go	95.80% <0.00%> (-1.20%)	⬇️
plugin/storage/badger/spanstore/reader.go	95.50% <0.00%> (-0.71%)	⬇️
plugin/storage/integration/integration.go	78.88% <0.00%> (-0.40%)	⬇️

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[joe-elliott]

Updated to accept the SAMPLING_TYPE env var as a deprecated method of setting the sampling type and static as a deprecated value.

Preferred is SAMPLING_CONFIG_TYPE and file.

$ SAMPLING_TYPE=static go run ./cmd/collector
2021/10/04 15:58:17 maxprocs: Leaving GOMAXPROCS=8: CPU quota undefined
WARNING: Using deprecated 'SAMPLING_TYPE' env var. Please switch to 'SAMPLING_CONFIG_TYPE'.
WARNING: Using deprecated 'static' value for SAMPLING_CONFIG_TYPE. Please switch to 'file'.
{"level":"info","ts":1633377497.5333173,"caller":"flags/service.go:117","msg":"Mounting metrics handler on admin server","route":"/metrics"}
...

[yurishkuro]

it might be even cleaner if you check for and override static in getStrategyStoreTypeFromEnv, because there was no release where SAMPLING_CONFIG_TYPE=static was valid.