StackExchange Redis: Provide 'db.system' on Span creation

[Austin-Tan]

Fixes #1971 for Redis -- The issue was fixed for SQL instrumentation library by @johnduhart in #1979 , and this PR is largely based on his work.

Changes

Since we always attach the tag KeyVal ("db.system", "redis"), statically instantiating it is worth the small cost, and allows samplers to check for this tag when deciding to sample or not

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@30b7dcf). Click here to learn what that means.
The diff coverage is 100.00%.

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Impacted Files	Coverage Δ
...mentation/RedisProfilerEntryToActivityConverter.cs	94.59% <100.00%> (ø)
...ngeRedis/StackExchangeRedisCallsInstrumentation.cs	96.82% <100.00%> (ø)

[Austin-Tan]

This collection is in the main RedisInstrumentation.cs file, as this is where we stored other static variables,
but I believe those were all only used in the class itself. We could alternatively put it in the Converter class where it's used, which is static (Does having it in static save memory? Most people are only instantiating one copy of the RedisInstrumentation.cs class anyway)