Required for all PRs:

• Updated associated README.md.
• Wrote appropriate unit tests.

resolves #9127

Fixed an incorrect mapping inside the nfsclient plugin input.

In the Init function of the nfsclient plugin, nfs3Ops and nfs4Ops is prepared in a local map inside but never attached to the underlying nfsclient struct.

Passing an empty map to

		if (version == "3" && n.nfs3Ops[first]) || (version == "4" && n.nfs4Ops[first]) {
			tags["operation"] = first
			if len(nline) <= len(nfsopFields) {
				for i, t := range nline {
					fields[nfsopFields[i]] = t
				}
				acc.AddFields("nfs_ops", fields, tags)
			}
		}

Result in n.nfs3Ops[first] or n.nfs4Ops[first] to be false, and this branch of the code is never reached.