[snmp] fix situation when ont requested oid is prefix for another requested oid

[xkrt]

I was faced with the following situation, my simplified conf:

metrics:
  - OID: 1.2.3
    name: first
  - OID: 1.2.3.4.5
    name: second

As you see OID of first is prefix of second OID. Without my bug fix both metrics get the value of first (since second have prefix 1.2.3).

[xkrt]

/сс @bunelr @truthbk @yannmh

[yannmh]

That's certainly an issue, thanks a lot for fixing it @xkrt !
We'll review and merge your fix for the upcoming 5.7.0 release.

[yannmh]

Looks great @xkrt. Would you mind squashing your commits so we can merge your contribution ?

Thanks again.

[xkrt]

@yannmh done

[yannmh]

Thanks. Let's merge it 🚢