Clarify faas.executions description in FaaS Metrics Spec

[pichlermarc]

What are you trying to achieve?

Currently the metric instrument with the name faas.executions in the FaaS Metrics Spec is described as

"Number of successful invocations."

which does not clearly state if it refers to no-error invocations or successful invocations. The goal of this issue is to have a clear description for faas.executions.

Additional context:

unrelated to this PR. I wonder if successful invocations here refer to no-error invocation or the fact it was invoked successfully? It deserves clarification

Originally posted by @SergeyKanzhelev in #2199 (comment)