otlp: Missing truncation for slice of strings attributes when converting to labels

[carsonip]

APM Server version (apm-server version): apm-data main

Description of the problem including expected versus actual behavior:

elastic/apm-data#346 seem to have introduced a regression, such that strings within slices are not truncated when converting otlp attributes to labels.

• Handling for strings: https://github.com/elastic/apm-data/blob/8e3a1a7610074fb5dfa7f794b1e1b3e3dca5857e/input/otlp/metadata.go#L501
• Handling for slice of strings: https://github.com/elastic/apm-data/blob/8e3a1a7610074fb5dfa7f794b1e1b3e3dca5857e/input/otlp/metadata.go#L519

[ericywl]

Fixed in #15702