[opentelemetry-collector] Fix opamp extension

[povilasv]

Currently when using preset we fail with:

2024/09/23 07:15:45 collector server run finished with error: failed to get config: cannot unmarshal the configuration: decoding failed due to the following error(s):

error decoding 'extensions': error reading configuration for "opamp": decoding failed due to the following error(s):

'agent_description.non_identifying_attributes' expected a map, got 'slice'

due to bad template generation