Redis:Omit command arguments from span.resource by default

[marcotc]

For release notes

The Datadog Agent removes command arguments from the resource name. However there are cases, like compressed keys, where this obfuscation cannot correctly remove command arguments. To safeguard that situation, the resource name set by the tracer will only be the command (e.g. SET) with no arguments. To retain the previous behavior and keep arguments in the span resource, with the potential risk of some command arguments not being fully obfuscated, set DD_REDIS_COMMAND_ARGS=true or option c.instrument :redis, command_args: true.

What does this PR do?

This PR changes the value of span.resource for the Redis tracing instrumentation to only capture the command name by default (e.g. GET, BLPOP).

It's possible to revert back to the previous default, capturing command arguments, with the environment variable DD_REDIS_COMMAND_ARGS or option command_args.

Motivation:

Redis command arguments can contain sensitive information and thus must not be captured by default.

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

Merging #3235 (47d136c) into master (4d22403) will increase coverage by 0.00%.
Report is 3 commits behind head on master.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3235   +/-   ##
=======================================
  Coverage   98.22%   98.22%           
=======================================
  Files        1252     1252           
  Lines       72201    72173   -28     
  Branches     3353     3348    -5     
=======================================
- Hits        70917    70890   -27     
+ Misses       1284     1283    -1     

Files	Coverage Δ
...og/tracing/contrib/redis/configuration/settings.rb	100.00% <100.00%> (ø)
...b/datadog/tracing/contrib/redis/instrumentation.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/contrib/redis/tags.rb	100.00% <100.00%> (ø)
.../datadog/tracing/contrib/redis/trace_middleware.rb	100.00% <100.00%> (ø)
.../datadog/tracing/contrib/rails/redis_cache_spec.rb	99.23% <100.00%> (ø)
...adog/tracing/contrib/redis/instrumentation_spec.rb	100.00% <ø> (ø)
spec/datadog/tracing/contrib/redis/redis_spec.rb	100.00% <100.00%> (ø)
...c/datadog/tracing/contrib/redis/shared_examples.rb	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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