VRL compiler improvements

[jszwedko]

Summary

VRL is a cornerstone of Vector. As such, we'd like to spend some time on compiler improvements to improve the UX.

Requirements

• VRL can be used to manipulate metric names and values #13031
• Metrics can be emitted from VRL #13032
• VRL doesn't correctly handle types during conditional assignment #11773
• Wrap VRL program in "block" expression to share code/performance characteristics #12686

[JeanMertz]

Closing this issue, as all listed items have been either solved, or moved into dedicated issues, tracked in this Compiler Improvements milestone.