RFR: 8251: Add missing hook in plugin from Jolokia discovery [v2]

[Martin Skarsaune]

> This got left out of #570 . All the test runs fine, but the mechansim was not properly plugged in :facepalm: 
> cc: @aptmac

Martin Skarsaune has updated the pull request incrementally with one additional commit since the last revision:

  JMC-8251: Adjust indentation in plugin.xml

-------------

Changes:
  - all: https://git.openjdk.org/jmc/pull/583/files
  - new: https://git.openjdk.org/jmc/pull/583/files/fb70cb4b..c048833f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmc&pr=583&range=01
 - incr: https://webrevs.openjdk.org/?repo=jmc&pr=583&range=00-01

  Stats: 18 lines in 1 file changed: 0 ins; 0 del; 18 mod
  Patch: https://git.openjdk.org/jmc/pull/583.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/583/head:pull/583

PR: https://git.openjdk.org/jmc/pull/583