Add description to force-merge tasks

[centic9]

This is related to #15975 by providing at least some description for running force-merge tasks and thus allows to see which force-merges on which indices are running and which settings were provided.

With this PR applied, the Task API will look something like the following for a "GET /_tasks?detailed":

        "bWByk2_lTGKufmq24Inu9g:418" : {
           "node" : "bWByk2_lTGKufmq24Inu9g",
           "action" : "indices:admin/forcemerge",
           "id" : 418,
           "headers" : {},
           "cancellable" : false,
           "running_time_in_nanos" : 161112867379,
           "description" : "Force-merge indices[twitter], maxSegments[-1], onlyExpungeDeletes[false], flush[true]",
           "start_time_in_millis" : 1555624171922,
           "type" : "transport"
        }

This does not provide progress-information, which would be very helpful for long-running merges to see how much longer they will approximately run, but this seems to be non-trivial to add, at least I could not find an easy way (hints for adding this are appreciated if there is a fairly simple approach for doing it!).

[elasticmachine]

Pinging @elastic/es-core-features

[martijnvg]

@elasticmachine test this please

[martijnvg]

Thanks @centic9, this is a very useful enhancement.

[martijnvg]

@centic9 I left one minor comment, do you want to address that?
Otherwise this looks good to me.

[javanna]

test this please

[javanna]

@elasticmachine update branch

[javanna]

retest this please

[javanna]

Thanks for your contribution and for addressing your PR based on the feedback @centic9 ! One more thing I would do is add a test so that we make sure we don't change this. A unit test that verifies the output of the new toString method would be enough for instance. Otherwise we may end up changing that one day without realizing what effect such change has.

[centic9]

Thanks for the response @javanna, I have now added a test which verifies the contents of getDescription() with various different values.

[martijnvg]

@elasticmachine test this please

[martijnvg]

@centic9 Thanks for updating the PR and apologies for not getting back to you.