node: default OpenTelemetry SampleRatio to 1.0

[barnabasbusa]

Summary

The --rpc.telemetry.sample-ratio flag declares Value: 1.0 and geth --help advertises (default: 1). In practice, however, omitting the flag produces a sample ratio of 0, causing sdktrace.TraceIDRatioBased(0) to drop 100% of spans. Users who enable --rpc.telemetry see the OpenTelemetry trace export enabled log line and a clean startup, but no traces ever leave the process.

The root cause is the interaction between two pieces of code:

1. cmd/utils/flags.go:setOpenTelemetry (added in cmd/utils: guard SampleRatio flag with IsSet check #34062) only copies the flag value when ctx.IsSet(...) returns true:

   if ctx.IsSet(RPCTelemetrySampleRatioFlag.Name) {
       tcfg.SampleRatio = ctx.Float64(RPCTelemetrySampleRatioFlag.Name)
   }

   That is the right pattern for "don't clobber a config-file value with the CLI default," but it implies that something else must initialise the field when neither source sets it.

2. node/defaults.go:DefaultConfig never initialises OpenTelemetry.SampleRatio, leaving it at the float64 zero value.

The result for the common CLI-only user (no TOML config) is SampleRatio = 0 → every span is silently dropped, despite the documented default of 1.

Change

Seed OpenTelemetry: OpenTelemetryConfig{SampleRatio: 1.0} in node.DefaultConfig so the documented default matches runtime behavior and the ctx.IsSet guard in setOpenTelemetry continues to do what it was designed to do.

Test plan

• Build geth from this branch and run with --rpc.telemetry --rpc.telemetry.endpoint=grpc://collector:4317 (no --rpc.telemetry.sample-ratio flag). Confirmed an OTLP gRPC connection is established and spans arrive at an OpenTelemetry collector.
• Re-tested with http://...:4318 to confirm both transports work without the explicit flag.
• Reproduced the broken behavior on plain master (no flag → no spans), and confirmed the fix restores expected behavior.
• Unit-test coverage in cmd/utils for the default-value path (not included; happy to add if reviewers prefer).

[fjl]

Please remove the tests. Also please use node.DefaultConfig.OpenTelemetry.SampleRatio to initialize the flag default value. This is what we usually do.

[barnabasbusa]

Removed the tests