Add option zipkin-max-span-batch-size (#8075)

### Problem
py_zipkin sends traces to Zipkin server in batches. Currently, the default value is set to 100 that can be not an optimal number for the number of spans in a trace for pants.

### Result
Adding the zipkin-max-span-batch-size option will allow finding the right batch size.

src/python/pants/reporting/reporting.py

@@ -59,7 +59,10 @@ def register_options(cls, register):
     register('--zipkin-trace-v2', advanced=True, type=bool, default=False,
               help='If enabled, the zipkin spans are tracked for v2 engine execution progress.')
     register('--zipkin-service-name-prefix', advanced=True, default='pants',
-      help='The prefix for service name for Zipkin spans.')
+              help='The prefix for service name for Zipkin spans.')
+    register('--zipkin-max-span-batch-size', advanced=True, type=int, default=100,
+              help='Spans in a Zipkin trace are sent to the Zipkin server in batches.' 
+                   'zipkin-max-span-batch-size sets the max size of one batch.')
 
   def initialize(self, run_tracker, all_options, start_time=None):
     """Initialize with the given RunTracker.
@@ -104,6 +107,7 @@ def initialize(self, run_tracker, all_options, start_time=None):
     service_name_prefix = self.get_options().zipkin_service_name_prefix
     if "{}" not in service_name_prefix:
       service_name_prefix = service_name_prefix + "/{}"
+    max_span_batch_size = int(self.get_options().zipkin_max_span_batch_size)
 
     if zipkin_endpoint is None and trace_id is not None and parent_id is not None:
       raise ValueError(
@@ -132,7 +136,8 @@ def initialize(self, run_tracker, all_options, start_time=None):
     if zipkin_endpoint is not None:
       zipkin_reporter_settings = ZipkinReporter.Settings(log_level=Report.INFO)
       zipkin_reporter = ZipkinReporter(
-        run_tracker, zipkin_reporter_settings, zipkin_endpoint, trace_id, parent_id, sample_rate, service_name_prefix
+        run_tracker, zipkin_reporter_settings, zipkin_endpoint, trace_id, parent_id, sample_rate,
+        service_name_prefix, max_span_batch_size
       )
       report.add_reporter('zipkin', zipkin_reporter)
 

src/python/pants/reporting/zipkin_reporter.py

@@ -39,7 +39,8 @@ class ZipkinReporter(Reporter):
     Reporter that implements Zipkin tracing.
   """
 
-  def __init__(self, run_tracker, settings, endpoint, trace_id, parent_id, sample_rate, service_name_prefix):
+  def __init__(self, run_tracker, settings, endpoint, trace_id, parent_id, sample_rate,
+                service_name_prefix, max_span_batch_size):
     """
     When trace_id and parent_id are set a Zipkin trace will be created with given trace_id
     and parent_id. If trace_id and parent_id are set to None, a trace_id will be randomly
@@ -51,6 +52,10 @@ def __init__(self, run_tracker, settings, endpoint, trace_id, parent_id, sample_
     :param string trace_id: The overall 64 or 128-bit ID of the trace. May be None.
     :param string parent_id: The 64-bit ID for a parent span that invokes Pants. May be None.
     :param float sample_rate: Rate at which to sample Zipkin traces. Value 0.0 - 100.0.
+    :param string service_name_prefix: Prefix for service name.
+    :param int max_span_batch_size: Spans in a trace are sent in batches,
+           max_span_batch_size defines max size of one batch.
+
     """
     super().__init__(run_tracker, settings)
     # Create a transport handler
@@ -62,6 +67,7 @@ def __init__(self, run_tracker, settings, endpoint, trace_id, parent_id, sample_
     self.tracer = get_default_tracer()
     self.run_tracker = run_tracker
     self.service_name_prefix = service_name_prefix
+    self.max_span_batch_size = max_span_batch_size
 
   def start_workunit(self, workunit):
     """Implementation of Reporter callback."""
@@ -105,11 +111,12 @@ def start_workunit(self, workunit):
         self.parent_id = zipkin_attrs.span_id
 
       span = local_tracer.zipkin_span(
-        service_name=self.service_name_prefix.format(service_name),
+        service_name=self.service_name_prefix.format("main"),
         span_name=workunit.name,
         transport_handler=self.handler,
         encoding=Encoding.V1_THRIFT,
         zipkin_attrs=zipkin_attrs,
+        max_span_batch_size=self.max_span_batch_size,
       )
     else:
       # If start_workunit is called from a new thread local_tracer doesn't have zipkin attributes.
@@ -128,7 +135,7 @@ def start_workunit(self, workunit):
       span.zipkin_attrs = span.zipkin_attrs._replace(
         parent_span_id=workunit.parent.zipkin_span.zipkin_attrs.span_id
       )
-      span.service_name = "pants background workunit"
+      span.service_name = self.service_name_prefix.format("background")
 
     # Goals and tasks save their start time at the beginning of their run.
     # This start time is passed to workunit, because the workunit may be created much later.