feat(telemetry): specify bucket boundaries for GenAI metrics

Specify explicit bucket boundaries for GenAI metrics histograms to align with OTel semconv updates:
- open-telemetry/semantic-conventions-genai#201
- open-telemetry/semantic-conventions-genai#202
- open-telemetry/semantic-conventions-genai#203

PiperOrigin-RevId: 938553240

Author: google-genai-bot

src/google/adk/telemetry/_metrics.py

@@ -45,26 +45,102 @@
     "gen_ai.agent.invocation.duration",
     unit="s",
     description="Duration of agent invocations.",
+    explicit_bucket_boundaries_advisory=[
+        0.1,
+        0.2,
+        0.4,
+        0.8,
+        1.6,
+        3.2,
+        6.4,
+        12.8,
+        25.6,
+        51.2,
+        102.4,
+        204.8,
+        409.6,
+    ],
 )
 _tool_execution_duration = meter.create_histogram(
     "gen_ai.tool.execution.duration",
     unit="s",
     description="Duration of tool executions.",
+    explicit_bucket_boundaries_advisory=[
+        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,
+    ],
 )
 _agent_request_size = meter.create_histogram(
     "gen_ai.agent.request.size",
     unit="By",
     description="Size of agent requests.",
+    explicit_bucket_boundaries_advisory=[
+        1,
+        4,
+        16,
+        64,
+        256,
+        1024,
+        4096,
+        16384,
+        65536,
+        262144,
+        1048576,
+        4194304,
+        16777216,
+        67108864,
+    ],
 )
 _agent_response_size = meter.create_histogram(
     "gen_ai.agent.response.size",
     unit="By",
     description="Size of agent responses.",
+    explicit_bucket_boundaries_advisory=[
+        1,
+        4,
+        16,
+        64,
+        256,
+        1024,
+        4096,
+        16384,
+        65536,
+        262144,
+        1048576,
+        4194304,
+        16777216,
+        67108864,
+    ],
 )
 _agent_workflow_steps = meter.create_histogram(
     "gen_ai.agent.workflow.steps",
     unit="1",
     description="Length of agentic workflow (# of events).",
+    explicit_bucket_boundaries_advisory=[
+        1,
+        2,
+        4,
+        8,
+        16,
+        32,
+        64,
+        128,
+        256,
+        512,
+        1024,
+    ],
 )
 _client_operation_duration = (
     gen_ai_metrics.create_gen_ai_client_operation_duration(meter)