I'm writing scripts to collect usage data from ArcGIS Server in order to generate reports. I used this Esri suport page to develop such scripts.
For simplicity, suppose the following json as my REST data to create an usage report:
<SPAN class="punctuation token">{</SPAN>
<SPAN class="string token">"reportname"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="string token">"my_debug_report"</SPAN><SPAN class="punctuation token">,</SPAN>
<SPAN class="string token">"since"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="string token">"LAST_DAY"</SPAN><SPAN class="punctuation token">,</SPAN>
<SPAN class="string token">"queries"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="punctuation token">{</SPAN>
<SPAN class="string token">"metrics"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">"RequestCount"</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"ServiceActiveInstances"</SPAN><SPAN class="punctuation token">]</SPAN><SPAN class="punctuation token">,</SPAN>
<SPAN class="string token">"resourceURIs"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">"services/MOSAICO/MOSAICO_ORTOFOTO_OESTE_2010.ImageServer"</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="punctuation token">}</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="punctuation token">}</SPAN><SPAN class="line-numbers-rows"><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN></SPAN>I receive then the following data:
| Timestamp | RequestCount of services/MOSAICO/MOSAICO_ORTOFOTO_OESTE_2010.ImageServer | ServiceActiveInstances of services/MOSAICO/MOSAICO_ORTOFOTO_OESTE_2010.ImageServer |
| 1477317600000 | 15 | NO SUCH METRIC SUPPORTED: ServiceActiveInstances |
| 1477319400000 | 19 | NO SUCH METRIC SUPPORTED: ServiceActiveInstances |
| 1477321200000 | 0 | NODATA |
| 1477323000000 | 0 | NODATA |
| 1477324800000 | 0 | NODATA |
| 1477326600000 | 0 | NODATA |
| 1477328400000 | 0 | NODATA |
| 1477330200000 | 42 | NO SUCH METRIC SUPPORTED: ServiceActiveInstances |
| 1477332000000 | 0 | NODATA |
| 1477333800000 | 36 | NO SUCH METRIC SUPPORTED: ServiceActiveInstances |
| 1477335600000 | 0 | NODATA |
| 1477337400000 | 0 | NODATA |
| 1477339200000 | 0 | NODATA |
| 1477341000000 | 32 | NO SUCH METRIC SUPPORTED: ServiceActiveInstances |
| 1477342800000 | 0 | NODATA |
| 1477344600000 | 0 | NODATA |
| 1477346400000 | 0 | NODATA |
| 1477348200000 | 0 | NODATA |
| 1477350000000 | 0 | NODATA |
| 1477351800000 | 0 | NODATA |
| 1477353600000 | 0 | NODATA |
| 1477355400000 | 0 | NODATA |
| 1477357200000 | 0 | NODATA |
| 1477359000000 | 0 | NODATA |
| 1477360800000 | 0 | NODATA |
| 1477362600000 | 0 | NODATA |
| 1477364400000 | 0 | NODATA |
| 1477366200000 | 0 | NODATA |
| 1477368000000 | 0 | NODATA |
| 1477369800000 | 0 | NODATA |
| 1477371600000 | 0 | NODATA |
| 1477373400000 | 0 | NODATA |
| 1477375200000 | 0 | NODATA |
| 1477377000000 | 0 | NODATA |
| 1477378800000 | 0 | NODATA |
| 1477380600000 | 0 | NODATA |
| 1477382400000 | 0 | NODATA |
| 1477384200000 | 0 | NODATA |
| 1477386000000 | 0 | NODATA |
| 1477387800000 | 0 | NODATA |
| 1477389600000 | 0 | NODATA |
| 1477391400000 | 0 | NODATA |
| 1477393200000 | 0 | NODATA |
| 1477395000000 | 0 | NODATA |
| 1477396800000 | 0 | NODATA |
| 1477398600000 | 0 | NODATA |
| 1477400400000 | 0 | NODATA |
| 1477402200000 | 0 | NODATA |
----
Updates from 2016/10/26
Using the server manager, I could generate the following report:

Despite of the Portuguese stuff, this report is from the same service witch I requested the json at the beginning of this post. In order to learn more about how the web manager do its job, I sniffed the network to capture the json request that generated this manage report and for my surprise I got this json:
<SPAN class="punctuation token">{</SPAN>
<SPAN class="string token">"reportname"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="number token">1477487802255</SPAN><SPAN class="punctuation token">,</SPAN>
<SPAN class="string token">"since"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="string token">"LAST_DAY"</SPAN><SPAN class="punctuation token">,</SPAN>
<SPAN class="string token">"queries"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="punctuation token">{</SPAN>
<SPAN class="string token">"resourceURIs"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">"services/SISGEOPOLIS/LinhaTrecho23.MapServer"</SPAN><SPAN class="punctuation token">]</SPAN><SPAN class="punctuation token">,</SPAN>
<SPAN class="string token">"metrics"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">"ServiceRunningInstancesMax"</SPAN><SPAN class="punctuation token">]</SPAN>
<SPAN class="punctuation token">}</SPAN><SPAN class="punctuation token">]</SPAN><SPAN class="punctuation token">,</SPAN>
<SPAN class="string token">"metadata"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">{</SPAN>
<SPAN class="string token">"temp"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="keyword token">true</SPAN><SPAN class="punctuation token">,</SPAN>
<SPAN class="string token">"tempTimer"</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="number token">1477488074129</SPAN>
<SPAN class="punctuation token">}</SPAN>
<SPAN class="punctuation token">}</SPAN><SPAN class="line-numbers-rows"><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN><SPAN></SPAN></SPAN>A strange thing here. Please take attention to the metrics array: I don't even know about the ServiceRunningInstancesMax. This official esri doc page was supposed to mention that but it does not.
Question 1: since there were service usage, why is ServiceActiveInstances not supported ?
Question 2: Where can I find an up to date documentation about admin usage reports which mentions all metrics, including ServiceRunningInstancesMax ?