Adding or partially overriding attributes

[mechie]

Unsure whether I'm not spotting the right incantation/flag to do this, or it is a feature request.

Under the following circumstances, it would be useful to be able to append/override attributes instead of overwriting them:

1. When calling span end on a span that already has attributes:

   TEST_SOCK=$(mktemp -d)
   otel-cli span background \
       --name "test" \
       --sockdir "${TEST_SOCK}" \
       --attr "color=red,size=large" \
       &
   # ...
   otel-cli span end \
       --sockdir "${TEST_SOCK}" \
       --attr "weight=100,size=small"
   # Want span with attributes { color=red, size=small, weight=100 }

2. When calling any command with --attr when OTEL_CLI_ATTRIBUTES is non-empty:

   OTEL_CLI_ATTRIBUTES="color=red,size=large"
   otel-cli exec \
       --name "test" \
       --attr "weight=100,size=small" \
       work
   # Want span with attributes { color=red, size=small, weight=100 }

In OTel terms of other languages/instrumentations, it would give us Set Attributes-like capabilities. This could be an additional attribute argument (--add-attr and OTEL_CLI_ADDITIONAL_ATTRIBUTES?), or a flag (--preserve-attr and OTEL_CLI_PRESERVE_EXISTING_ATTRIBUTES?) modifying the behavior of --attr.