feat: O11Y-677 - Add session.id to all metrics (#279)

## Summary
This change ensures that all custom metrics recorded by the Android
observability SDK include the `session.id` attribute.

## Are there any deployment considerations?
This won't work for OTEL instrumentations like
`io.opentelemetry.okhttp-3.0` that automatically record metrics behind
the scenes without using our record methods. That edge case will be
tackled in https://launchdarkly.atlassian.net/browse/O11Y-711

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Appends `session.id` to attributes for all recorded metrics via a new
`Attributes.addSessionId()` helper.
> 
> - **Metrics**:
> - Append `session.id` to attributes in `recordMetric`, `recordCount`,
`recordIncr`, `recordHistogram`, and `recordUpDownCounter` by using
`metric.attributes.addSessionId()` in `InstrumentationManager.kt`.
> - Add `Attributes.addSessionId()` helper and `SESSION_ID_ATTRIBUTE =
"session.id"` constant.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
3b48286. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: agrognetti

sdk/@launchdarkly/observability-android/lib/src/main/kotlin/com/launchdarkly/observability/client/InstrumentationManager.kt

@@ -277,37 +277,37 @@ class InstrumentationManager(
         val gauge = gaugeCache.getOrPut(metric.name) {
             otelMeter.gaugeBuilder(metric.name).build()
         }
-        gauge.set(metric.value, metric.attributes)
+        gauge.set(metric.value, metric.attributes.addSessionId())
     }
 
     fun recordCount(metric: Metric) {
         // TODO: handle double casting to long better
         val counter = counterCache.getOrPut(metric.name) {
             otelMeter.counterBuilder(metric.name).build()
         }
-        counter.add(metric.value.toLong(), metric.attributes)
+        counter.add(metric.value.toLong(), metric.attributes.addSessionId())
     }
 
     fun recordIncr(metric: Metric) {
         val counter = counterCache.getOrPut(metric.name) {
             otelMeter.counterBuilder(metric.name).build()
         }
         // It increments the value until the metric is exported, then it’s reset.
-        counter.add(1, metric.attributes)
+        counter.add(1, metric.attributes.addSessionId())
     }
 
     fun recordHistogram(metric: Metric) {
         val histogram = histogramCache.getOrPut(metric.name) {
             otelMeter.histogramBuilder(metric.name).build()
         }
-        histogram.record(metric.value, metric.attributes)
+        histogram.record(metric.value, metric.attributes.addSessionId())
     }
 
     fun recordUpDownCounter(metric: Metric) {
         val upDownCounter = upDownCounterCache.getOrPut(metric.name) {
             otelMeter.upDownCounterBuilder(metric.name).build()
         }
-        upDownCounter.add(metric.value.toLong(), metric.attributes)
+        upDownCounter.add(metric.value.toLong(), metric.attributes.addSessionId())
     }
 
     fun recordLog(message: String, severity: Severity, attributes: Attributes) {
@@ -385,12 +385,15 @@ class InstrumentationManager(
         }
     }
 
+    private fun Attributes.addSessionId() = this.toBuilder().put(SESSION_ID_ATTRIBUTE, otelRUM.rumSessionId).build()
+
     companion object {
         private const val METRICS_PATH = "/v1/metrics"
         private const val LOGS_PATH = "/v1/logs"
         private const val TRACES_PATH = "/v1/traces"
         private const val INSTRUMENTATION_SCOPE_NAME = "com.launchdarkly.observability"
         const val ERROR_SPAN_NAME = "highlight.error"
+        const val SESSION_ID_ATTRIBUTE = "session.id"
         private const val BATCH_MAX_QUEUE_SIZE = 100
         private const val BATCH_SCHEDULE_DELAY_MS = 1000L
         private const val BATCH_EXPORTER_TIMEOUT_MS = 5000L