docs: update pre-fork server guidance

docs/examples/fork-process-model/README.rst

@@ -1,14 +1,21 @@
 Working With Fork Process Models
 ================================
 
-The `BatchSpanProcessor` is not fork-safe and doesn't work well with application servers
-(Gunicorn, uWSGI) which are based on the pre-fork web server model. The `BatchSpanProcessor`
-spawns a thread to run in the background to export spans to the telemetry backend. During the fork, the child
-process inherits the lock which is held by the parent process and deadlock occurs. We can use fork hooks to
-get around this limitation of the span processor.
-
-Please see http://bugs.python.org/issue6721 for the problems about Python locks in (multi)threaded
-context with fork.
+The tracing and logging batch processors reinitialize their background worker
+state after a process forks, so they can be created before application servers
+such as Gunicorn and uWSGI fork their workers. Metrics aggregation state is not
+safe to share across forked workers, however. Applications using metrics with a
+pre-fork server should create their ``MeterProvider`` and metric readers in each
+child process.
+
+The examples below initialize tracing and metrics together in a fork hook. Only
+the metrics setup requires this placement, but keeping the providers together
+gives every worker a consistent resource and avoids sharing metric state. An
+application that emits only traces or logs can initialize its providers before
+the server forks.
+
+Please see https://bugs.python.org/issue6721 for general problems involving
+Python locks in multithreaded programs that fork.
 
 The source code for the examples with Flask app are available :scm_web:`here <docs/examples/fork-process-model/>`.
 
@@ -17,8 +24,11 @@ Gunicorn post_fork hook
 
 .. code-block:: python
 
-    from opentelemetry import trace
+    from opentelemetry import metrics, trace
+    from opentelemetry.exporter.otlp.proto.grpc.metric_exporter import OTLPMetricExporter
     from opentelemetry.exporter.otlp.proto.grpc.trace_exporter import OTLPSpanExporter
+    from opentelemetry.sdk.metrics import MeterProvider
+    from opentelemetry.sdk.metrics.export import PeriodicExportingMetricReader
     from opentelemetry.sdk.resources import Resource
     from opentelemetry.sdk.trace import TracerProvider
     from opentelemetry.sdk.trace.export import BatchSpanProcessor
@@ -37,6 +47,17 @@ Gunicorn post_fork hook
         )
         trace.get_tracer_provider().add_span_processor(span_processor)
 
+        metrics.set_meter_provider(
+            MeterProvider(
+                resource=resource,
+                metric_readers=[
+                    PeriodicExportingMetricReader(
+                        OTLPMetricExporter(endpoint="http://localhost:4317")
+                    )
+                ],
+            )
+        )
+
 
 uWSGI postfork decorator
 ------------------------
@@ -45,15 +66,18 @@ uWSGI postfork decorator
 
     from uwsgidecorators import postfork
 
-    from opentelemetry import trace
+    from opentelemetry import metrics, trace
+    from opentelemetry.exporter.otlp.proto.grpc.metric_exporter import OTLPMetricExporter
     from opentelemetry.exporter.otlp.proto.grpc.trace_exporter import OTLPSpanExporter
+    from opentelemetry.sdk.metrics import MeterProvider
+    from opentelemetry.sdk.metrics.export import PeriodicExportingMetricReader
     from opentelemetry.sdk.resources import Resource
     from opentelemetry.sdk.trace import TracerProvider
     from opentelemetry.sdk.trace.export import BatchSpanProcessor
 
 
     @postfork
-    def init_tracing():
+    def init_telemetry():
         resource = Resource.create(attributes={
             "service.name": "api-service"
         })
@@ -63,3 +87,14 @@ uWSGI postfork decorator
             OTLPSpanExporter(endpoint="http://localhost:4317")
         )
         trace.get_tracer_provider().add_span_processor(span_processor)
+
+        metrics.set_meter_provider(
+            MeterProvider(
+                resource=resource,
+                metric_readers=[
+                    PeriodicExportingMetricReader(
+                        OTLPMetricExporter(endpoint="http://localhost:4317")
+                    )
+                ],
+            )
+        )

docs/examples/fork-process-model/flask-uwsgi/app.py

@@ -5,11 +5,16 @@
 from flask import request
 from uwsgidecorators import postfork
 
-from opentelemetry import trace
+from opentelemetry import metrics, trace
+from opentelemetry.exporter.otlp.proto.grpc.metric_exporter import (
+    OTLPMetricExporter,
+)
 from opentelemetry.exporter.otlp.proto.grpc.trace_exporter import (
     OTLPSpanExporter,
 )
 from opentelemetry.instrumentation.flask import FlaskInstrumentor
+from opentelemetry.sdk.metrics import MeterProvider
+from opentelemetry.sdk.metrics.export import PeriodicExportingMetricReader
 from opentelemetry.sdk.resources import Resource
 from opentelemetry.sdk.trace import TracerProvider
 from opentelemetry.sdk.trace.export import BatchSpanProcessor
@@ -22,7 +27,7 @@
 
 
 @postfork
-def init_tracing():
+def init_telemetry():
     resource = Resource.create(attributes={"service.name": "api-service"})
 
     trace.set_tracer_provider(TracerProvider(resource=resource))
@@ -31,6 +36,14 @@ def init_tracing():
     span_processor = BatchSpanProcessor(OTLPSpanExporter(endpoint="http://localhost:4317", insecure=True))
     trace.get_tracer_provider().add_span_processor(span_processor)
 
+    reader = PeriodicExportingMetricReader(OTLPMetricExporter(endpoint="http://localhost:4317", insecure=True))
+    metrics.set_meter_provider(
+        MeterProvider(
+            resource=resource,
+            metric_readers=[reader],
+        )
+    )
+
 
 def fib_slow(n):
     if n <= 1: