fix(telemetry): add 0 bucket boundary to per-invocation call count histograms

Include 0 in the explicit bucket boundaries for the inference call count and
tool call count histograms so invocations with 0 model/tool calls and 1
mode/tool calls can be differentiated.

Co-authored-by: Max Ind <maxind@google.com>
PiperOrigin-RevId: 943888837

Author: RKest

src/google/adk/telemetry/_metrics.py

@@ -99,6 +99,7 @@
     unit="1",
     description="Number of inference (model) calls per agent invocation.",
     explicit_bucket_boundaries_advisory=[
+        0,
         1,
         2,
         3,
@@ -118,6 +119,7 @@
     unit="1",
     description="Number of tool calls per agent invocation.",
     explicit_bucket_boundaries_advisory=[
+        0,
         1,
         2,
         3,