[chore] Downgrade OTEL libraries to 1.20 (#2546)

For some reason httpconv seems to have disappeared from 1.21, which
results in a 1.21 runtime with 1.20 semconv/httpconv which seems to
break some things.

For now, this rolls the OTEL dependencies back to 1.20 which should fix
the observability issues. We'll need to take a look at how to upgrade
safely and correctly in the future.

Relates to #2503.

vendor/go.opentelemetry.io/otel/versions.yaml

@@ -14,12 +14,13 @@
 
 module-sets:
   stable-v1:
-    version: v1.21.0
+    version: v1.20.0
     modules:
       - go.opentelemetry.io/otel
       - go.opentelemetry.io/otel/bridge/opentracing
       - go.opentelemetry.io/otel/bridge/opentracing/test
       - go.opentelemetry.io/otel/example/dice
+      - go.opentelemetry.io/otel/example/fib
       - go.opentelemetry.io/otel/example/namedtracer
       - go.opentelemetry.io/otel/example/otel-collector
       - go.opentelemetry.io/otel/example/passthrough
@@ -34,12 +35,14 @@ module-sets:
       - go.opentelemetry.io/otel/sdk/metric
       - go.opentelemetry.io/otel/trace
   experimental-metrics:
-    version: v0.44.0
+    version: v0.43.0
     modules:
       - go.opentelemetry.io/otel/bridge/opencensus
       - go.opentelemetry.io/otel/bridge/opencensus/test
       - go.opentelemetry.io/otel/example/opencensus
       - go.opentelemetry.io/otel/example/prometheus
+      - go.opentelemetry.io/otel/example/view
+      - go.opentelemetry.io/otel/exporters/otlp/otlpmetric
       - go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc
       - go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp
       - go.opentelemetry.io/otel/exporters/prometheus