Align label behavior of median aggregation with that of other single metrics aggregations

[sophiec20]

Kibana version: 6.3.0

Original install method (e.g. download page, yum, from source, etc.): zip

Describe the bug:
The visualisation custom label for median is applied differently to the custom labels for mean etc

Steps to reproduce:

1. Create data table visualization
2. For a numeric value, pick mean, min, max and median
3. View results of visualization

Expected behavior:
The column heading for median is written as "50th percentile of median". I would have expected this to be written just as "median" which would be similar behavior to "mean".

Screenshots (if relevant):

Errors in browser console (if relevant):
None

Provide logs and/or server output (if relevant):
N/a

Any additional context:

[timroes]

Since the media is just the 50th percentile, we are using the percentiles aggregation with 50th percentile, but that means that also the percentile labels will be used, which always prefix the percentile to the actual label, since you could potentially have multiple percentiles specified. So I consider this to be expected behavior at the moment, and switched the label from bug to enhancement, since I would totally agree, that it might be nicer if we would actually behave the same as with Average and I would also like to by default just having the label Median of processing_time_ms instead of 50th percentile of processing_time_ms.