Add description to metric

[MalloZup]

description:

this add metric like :

# HELP saptune_note_enabled This metrics show with 1 all the enabled notes on the system
# TYPE saptune_note_enabled gauge
saptune_note_enabled{note_desc="Indexserver Crash During Startup due to Insufficient Shared Memory Segment",note_id="2534844"} 1
saptune_note_enabled{note_desc="Linux: User and system resource limits",note_id="1771258"} 1
saptune_note_enabled{note_desc="Optimizing the Network Configuration on HANA- and OS-Level",note_id="2382421"} 1
saptune_note_enabled{note_desc="SAP HANA DB: Recommended OS settings for SLES 15 / SLES for SAP Applications 15",note_id="2684254"} 1
saptune_note_enabled{note_desc="SAP memory management system for 64-bit Linux systems",note_id="941735"} 1
saptune_note_enabled{note_desc="SUSE LINUX Enterprise Server 15: Installation notes",note_id="2578899"} 1
saptune_note_enabled{note_desc="Setting Linux Kernel Parameter /proc/sys/vm/max_map_count on SAP HANA Systems",note_id="1980196"} 1

[MalloZup]

@stefanotorresi I'm merging this to master for rebuilding pkg. if you have any major concern on this PR feel free to comment here I will adress the comments on a later PR as follow-up, but given the "goals" and "rationale" of this exporter, I think we can be more flexibile

[stefanotorresi]

it's okay!