[tailsamplingprocessor] Fix the misleading decision timer latency metric (open-telemetry#39761)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

It will now measure the total time spent on one tick of the decision
timer. This is more useful, since the decision tick loop needs to run at
least once per second for the tsp to keep up with incoming traces.



<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes open-telemetry#38502

Author: Logiraptor

.chloggen/logiraptor_fix-decision-timer-latency-metric.yaml

@@ -0,0 +1,28 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: tailsamplingprocessor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix the misleading decision timer latency metric
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues:
+- 38502
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

processor/tailsamplingprocessor/processor.go

@@ -355,7 +355,6 @@ func (tsp *tailSamplingSpanProcessor) samplingPolicyOnTick() {
 
 		decision := tsp.makeDecision(id, trace, &metrics)
 
-		tsp.telemetry.ProcessorTailSamplingSamplingDecisionTimerLatency.Record(tsp.ctx, int64(time.Since(startTime)/time.Millisecond))
 		tsp.telemetry.ProcessorTailSamplingGlobalCountTracesSampled.Add(tsp.ctx, 1, decisionToAttribute[decision])
 
 		// Sampled or not, remove the batches
@@ -373,6 +372,7 @@ func (tsp *tailSamplingSpanProcessor) samplingPolicyOnTick() {
 		}
 	}
 
+	tsp.telemetry.ProcessorTailSamplingSamplingDecisionTimerLatency.Record(tsp.ctx, int64(time.Since(startTime)/time.Millisecond))
 	tsp.telemetry.ProcessorTailSamplingSamplingTracesOnMemory.Record(tsp.ctx, int64(tsp.numTracesOnMap.Load()))
 	tsp.telemetry.ProcessorTailSamplingSamplingTraceDroppedTooEarly.Add(tsp.ctx, metrics.idNotFoundOnMapCount)
 	tsp.telemetry.ProcessorTailSamplingSamplingPolicyEvaluationError.Add(tsp.ctx, metrics.evaluateErrorCount)