Minor improvements

- rename `MetricObserver::weakMap()` to `::destructors()` to better reflect usage`
- move `ReferenceCounter::acquire()` call to corresponding `MetricObserver::observe()` call for consistency
- cache instrumentation scope id in `Meter` to avoid repeated calls to `serialize()`
- remove obsolete instrument type check from `ViewProjection`, leftover from supporting aggregation per type

src/SDK/Metrics/Meter.php

@@ -37,6 +37,8 @@ final class Meter implements MeterInterface
     private MeterInstruments $instruments;
     private InstrumentationScopeInterface $instrumentationScope;
 
+    private ?string $instrumentationScopeId = null;
+
     /**
      * @param iterable<MetricSourceRegistryInterface&DefaultAggregationProviderInterface> $metricRegistries
      */
@@ -164,8 +166,8 @@ private function createSynchronousWriter($instrumentType, string $name, ?string
     {
         $instrument = new Instrument($instrumentType, $name, $unit, $description);
 
-        $instrumentationScopeId = self::instrumentationScopeId($this->instrumentationScope);
-        $instrumentId = self::instrumentId($instrument);
+        $instrumentationScopeId = $this->instrumentationScopeId($this->instrumentationScope);
+        $instrumentId = $this->instrumentId($instrument);
 
         $instruments = $this->instruments;
         if ($writer = $instruments->writers[$instrumentationScopeId][$instrumentId] ?? null) {
@@ -207,8 +209,8 @@ private function createAsynchronousObserver($instrumentType, string $name, ?stri
     {
         $instrument = new Instrument($instrumentType, $name, $unit, $description);
 
-        $instrumentationScopeId = self::instrumentationScopeId($this->instrumentationScope);
-        $instrumentId = self::instrumentId($instrument);
+        $instrumentationScopeId = $this->instrumentationScopeId($this->instrumentationScope);
+        $instrumentId = $this->instrumentId($instrument);
 
         $instruments = $this->instruments;
         if ($observer = $instruments->observers[$instrumentationScopeId][$instrumentId] ?? null) {
@@ -277,12 +279,12 @@ private function viewRegistrationRequests(Instrument $instrument, StalenessHandl
         }
     }
 
-    private static function instrumentationScopeId(InstrumentationScopeInterface $instrumentationScope): string
+    private function instrumentationScopeId(InstrumentationScopeInterface $instrumentationScope): string
     {
-        return serialize($instrumentationScope);
+        return $this->instrumentationScopeId ??= serialize($instrumentationScope);
     }
 
-    private static function instrumentId(Instrument $instrument): string
+    private function instrumentId(Instrument $instrument): string
     {
         return serialize($instrument);
     }

src/SDK/Metrics/MetricObserver/MultiObserver.php

@@ -45,7 +45,7 @@ public function cancel(int $token): void
         unset($this->callbacks[$token]);
     }
 
-    public function weakMap(): ArrayAccess
+    public function destructors(): ArrayAccess
     {
         return $this->weakMap ??= WeakMap::create();
     }

src/SDK/Metrics/MetricObserverInterface.php

@@ -7,6 +7,7 @@
 use ArrayAccess;
 use Closure;
 use OpenTelemetry\API\Metrics\ObserverInterface;
+use OpenTelemetry\SDK\Metrics\MetricObserver\CallbackDestructor;
 
 interface MetricObserverInterface
 {
@@ -19,5 +20,8 @@ public function has(int $token): bool;
 
     public function cancel(int $token): void;
 
-    public function weakMap(): ArrayAccess;
+    /**
+     * @return ArrayAccess<object, CallbackDestructor>
+     */
+    public function destructors(): ArrayAccess;
 }

src/SDK/Metrics/ObservableCallback.php

@@ -20,8 +20,6 @@ public function __construct(MetricObserverInterface $metricObserver, ReferenceCo
         $this->referenceCounter =  $referenceCounter;
         $this->token = $token;
         $this->callbackDestructor = $callbackDestructor;
-
-        $this->referenceCounter->acquire();
     }
 
     public function detach(): void

src/SDK/Metrics/ObservableInstrumentTrait.php

@@ -45,10 +45,11 @@ public function observe(callable $callback, bool $weaken = false): ObservableCal
 
         /** @psalm-var \Closure(ObserverInterface): void $callback */
         $token = $this->metricObserver->observe($callback);
+        $this->referenceCounter->acquire();
 
         $destructor = null;
         if ($object = $target) {
-            $destructor = $this->metricObserver->weakMap()[$object] ??= new CallbackDestructor($this->metricObserver, $this->referenceCounter);
+            $destructor = $this->metricObserver->destructors()[$object] ??= new CallbackDestructor($this->metricObserver, $this->referenceCounter);
             $destructor->tokens[$token] = $token;
         }
 

src/SDK/Metrics/View/ViewTemplate.php

@@ -4,8 +4,6 @@
 
 namespace OpenTelemetry\SDK\Metrics\View;
 
-use function assert;
-use function is_string;
 use OpenTelemetry\SDK\Metrics\AggregationInterface;
 use OpenTelemetry\SDK\Metrics\Instrument;
 use OpenTelemetry\SDK\Metrics\ViewProjection;
@@ -68,9 +66,6 @@ public function withAggregation(?AggregationInterface $aggregation): self
 
     public function project(Instrument $instrument): ViewProjection
     {
-        $instrumentType = $instrument->type;
-        assert(is_string($instrumentType));
-
         return new ViewProjection(
             $this->name ?? $instrument->name,
             $instrument->unit,