docs: record PDH counter types of all collectors for agents (#2713)

Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

AGENTS.md

@@ -186,6 +186,25 @@ go build -o windows_exporter.dev.exe ./cmd/windows_exporter/
 Adjust the collector list for the change. Leave the existing exporter instance
 running unless the user requests changes to it.
 
+To check a collector's metric types against Windows, list each counter of the
+PDH object with its type and raw value. Windows PowerShell 5.1 can do this
+through .NET; `RawValue` and `BaseValue` are what `pdh.CounterTypeRaw` returns
+as first and second value:
+
+```powershell
+$cat = [System.Diagnostics.PerformanceCounterCategory]'DirectoryServices'
+$cat.GetCounters('NTDS') | ForEach-Object {
+    $s = $_.NextSample()
+    '{0} | {1} | raw={2} base={3}' -f $_.CounterName, $_.CounterType, $s.RawValue, $s.BaseValue
+}
+```
+
+Every PDH-based collector except `exchange`, `smtp` and `vmware` has such a
+dump in the `AGENTS.md` of its directory, including roles that are rarely
+available on development machines (AD DS, AD CS, AD FS, DFSR, Hyper-V, IIS,
+SQL Server, NPS, RemoteFX sessions). Check a type or unit question there
+first.
+
 ## Dashboards, rules and runbooks
 
 Read [`contrib/mixin/README.md`](contrib/mixin/README.md) and