chore: add OTEL config to .env.example and docker-compose, remove fly.provider-b.toml

Adds commented-out OTEL configuration section to .env.example, adds
OTEL environment variables to docker-compose provider service, and
removes unused fly.provider-b.toml deployment config.

Author: AquiGorka

.env.example

@@ -27,6 +27,12 @@ SERVICE_AUTH_SECRET= # 32 random bytes, base64 encoded. Used for secure JWT midd
 CHALLENGE_TTL=900 #15m
 SESSION_TTL=21600 #6h
 
+# OPENTELEMETRY (opt-in, requires Jaeger or OTLP-compatible collector)
+# OTEL_DENO=true
+# OTEL_SERVICE_NAME=provider-platform
+# OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318
+# OTEL_EXPORTER_OTLP_PROTOCOL=http/protobuf
+
 # MEMPOOL CONFIGURATION
 MEMPOOL_SLOT_CAPACITY=100
 MEMPOOL_EXPENSIVE_OP_WEIGHT=10