Fix: Convert DSN to string in telemetry envelope header

Problem

When enabling log ingestion with config.enable_logs = true, the application crashes with the following error:

[Sentry::LogEventBuffer] Failed to send Sentry::LogEvent: the server responded with status 400
body: {"detail":"invalid event envelope","causes":["invalid envelope header","invalid type: map, expected a string at line 1 column 360"]}

Root Cause

The TelemetryEventBuffer#send_items method incorrectly passes the DSN as a Ruby object instead of a string:

envelope = Envelope.new(
  event_id: Sentry::Utils.uuid,
  sent_at: Sentry.utc_now.iso8601,
  dsn: @dsn,  # ← DSN is a Ruby object, not a string!
  sdk: Sentry.sdk_meta
)

When the envelope is serialized in Transport#serialize_envelope, it calls JSON.generate(envelope.headers), which fails because the DSN is a complex Ruby object (instance of Sentry::DSN), not a primitive JSON-serializable type.

The Sentry server expects all envelope header values to be strings, numbers, or booleans - not maps/objects.

Solution

Convert the DSN to a string using .to_s before adding it to the envelope header:

envelope = Envelope.new(
  event_id: Sentry::Utils.uuid,
  sent_at: Sentry.utc_now.iso8601,
  dsn: @dsn.to_s,  # ← Convert to string!
  sdk: Sentry.sdk_meta
)

Why this approach:

• This aligns with the existing implementation in Transport#envelope_from_event (line 122), which uses dsn: @dsn.to_s
• The DSN is included in envelope headers for other event types (events, transactions) - see transport_spec.rb tests
• Ensures consistency across all envelope types in the codebase
• Properly serializes to JSON while maintaining the DSN information in the envelope

Changes

1. sentry-ruby/lib/sentry/telemetry_event_buffer.rb: Convert DSN to string with .to_s in envelope header
2. sentry-ruby/spec/sentry/metrics_spec.rb: Fix test to expect string DSN instead of DSN object

Testing

1. Enable log ingestion: config.enable_logs = true
2. Configure log subscribers (e.g., ActiveRecord, ActionController)
3. Trigger a log event
4. Verify logs are successfully sent to Sentry without the 400 error

Affected Versions

This bug affects all versions with the telemetry event buffer, including:

• sentry-ruby 6.3.0+
• Any version with config.enable_logs or config.enable_metrics features

Related Files

• sentry-ruby/lib/sentry/telemetry_event_buffer.rb (fixed)
• sentry-ruby/lib/sentry/transport.rb (reference implementation at line 122)
• sentry-ruby/spec/sentry/transport_spec.rb (shows DSN as string in tests)