[Metric API] Instrument validation error handling

[cijothomas]

Noticed an inconsistency in the Metric API regarding the init and try_init methods for instrument initialization. The documentation for init says that it would panic when validation fails, while try_init is expected to return a Result, allowing for error handling.

However, it appears that neither method behaves as documented. Specifically, the SDK implementation seems to suppress the Err variant, invariably returning an Ok variant after logging an error, and continues to provide a valid instrument. This should be potentially be classified as a bug.

Proposing the following changes to address this:

• Modify the SDK implementation to return a NoOp instrument instead of an actual instrument when validation fails.

Additionally, need some inputs/suggestions of the below:

1. Considering that the Err variant is never returned, should we remove the try_init method?
2. Alternatively, should we retain both init and try_init, ensuring they adhere to the current documentation—where init panics on invalid input, and try_init returns a Result? This would place the onus on the user to either handle the results or accept the possibility of a panic.