refactor(openai): Use shared genai-utils helpers for metric creation

Move metric histogram creation to use create_duration_histogram,
create_token_histogram, and create_ttft_histogram from
opentelemetry.util.genai.instruments instead of defining bucket
boundaries and creating histograms inline.

This aligns with the pattern of keeping common metric definitions
in genai-utils rather than individual instrumentations.

Author: Nik-Reddy

instrumentation-genai/opentelemetry-instrumentation-openai-v2/src/opentelemetry/instrumentation/openai_v2/instruments.py

@@ -1,62 +1,13 @@
 from opentelemetry.metrics import Histogram, Meter
-from opentelemetry.semconv._incubating.metrics import gen_ai_metrics
 from opentelemetry.util.genai.instruments import (
-    GEN_AI_CLIENT_TIME_TO_FIRST_TOKEN,
-    GEN_AI_CLIENT_TIME_TO_FIRST_TOKEN_BUCKETS,
+    create_duration_histogram,
+    create_token_histogram,
+    create_ttft_histogram,
 )
 
-_GEN_AI_CLIENT_OPERATION_DURATION_BUCKETS = [
-    0.01,
-    0.02,
-    0.04,
-    0.08,
-    0.16,
-    0.32,
-    0.64,
-    1.28,
-    2.56,
-    5.12,
-    10.24,
-    20.48,
-    40.96,
-    81.92,
-]
-
-_GEN_AI_CLIENT_TOKEN_USAGE_BUCKETS = [
-    1,
-    4,
-    16,
-    64,
-    256,
-    1024,
-    4096,
-    16384,
-    65536,
-    262144,
-    1048576,
-    4194304,
-    16777216,
-    67108864,
-]
-
 
 class Instruments:
     def __init__(self, meter: Meter):
-        self.operation_duration_histogram: Histogram = meter.create_histogram(
-            name=gen_ai_metrics.GEN_AI_CLIENT_OPERATION_DURATION,
-            description="GenAI operation duration",
-            unit="s",
-            explicit_bucket_boundaries_advisory=_GEN_AI_CLIENT_OPERATION_DURATION_BUCKETS,
-        )
-        self.token_usage_histogram: Histogram = meter.create_histogram(
-            name=gen_ai_metrics.GEN_AI_CLIENT_TOKEN_USAGE,
-            description="Measures number of input and output tokens used",
-            unit="{token}",
-            explicit_bucket_boundaries_advisory=_GEN_AI_CLIENT_TOKEN_USAGE_BUCKETS,
-        )
-        self.ttft_histogram: Histogram = meter.create_histogram(
-            name=GEN_AI_CLIENT_TIME_TO_FIRST_TOKEN,
-            description="Time to generate first token for successful responses",
-            unit="s",
-            explicit_bucket_boundaries_advisory=GEN_AI_CLIENT_TIME_TO_FIRST_TOKEN_BUCKETS,
-        )
+        self.operation_duration_histogram: Histogram = create_duration_histogram(meter)
+        self.token_usage_histogram: Histogram = create_token_histogram(meter)
+        self.ttft_histogram: Histogram = create_ttft_histogram(meter)