feat: Remove trace key params -> add meta sample_key

[kentquirk]

Which problem is this PR solving?

Refinery had rules configuration for dynamic samplers to "AddSampleRateKeyToTrace" which would inject the sample rate key into a field of the user's choice. We also have, in the normal configuration "AddRuleReasonToTrace", which also injected that key but as part of the rule reason in meta.refinery.reason, causing people to have to write derived columns to pull it off.

This PR addresses both of those.

Short description of the changes

• Remove AddSampleRateKeyToTrace and AddSampleRateKeyToTraceField params from samplers
• Remove all the tests for it
• Modify the GetSampleRate function to return the reason and key separately (or a blank key)
• If the key isn't blank, add it to the spans under meta.refinery.sample_key
• Add tests for it

This looks big but it's a lot of small identical changes in multiple files.