BUG/MINOR: samples: add missing context names for sample fetch functions

BUG/MINOR: samples: add missing context names for sample fetch functions

In 2.4, two commits added support for supporting sample fetch calls from
new config and CLI contexts, but these were not added to the visibile
names, which may possibly cause "(null)" to appear in some error messages.
The commit in question were:
  db5e0dbea ("MINOR: sample: add a new CLI_PARSER context for samples")
  f9a7a8fd8 ("MINOR: sample: add a new CFG_PARSER context for samples")

This patch needs to be backported where these are present (2.4 and above).