set_propagation_headers overwrites existing baggage header instead of merging #2894

Issue Description

Sentry::Utils::HttpTracing#set_propagation_headers unconditionally overwrites all propagation headers on outgoing HTTP requests using req[k] = v. While this is fine for the Sentry-specific sentry-trace header, the baggage header is a W3C standard shared across multiple systems. Any pre-existing baggage entries (e.g. set by OpenTelemetry, application code, or other instrumentation) are silently discarded.

The affected code is in sentry-ruby/lib/sentry/utils/http_tracing.rb#L14-L16:

def set_propagation_headers(req)
  Sentry.get_trace_propagation_headers&.each { |k, v| req[k] = v }
end

Reproduction Steps

1. Set a custom baggage header on an outgoing HTTP request (e.g. routingKey=myvalue)
2. Ensure propagate_traces is enabled in Sentry config and the target URL matches trace_propagation_targets
3. Make the HTTP request using Net::HTTP, Faraday, or Excon
4. Inspect the outgoing request headers

require "net/http"

Sentry.init do |config|
  config.dsn = "https://key@sentry.io/1"
  config.traces_sample_rate = 1.0
  config.propagate_traces = true
end

uri = URI("https://example.com/api")
req = Net::HTTP::Get.new(uri)
req["baggage"] = "routingKey=myvalue,tenantId=123"

# After Sentry instruments this request, the baggage header
# will only contain sentry-* entries -- the original values are lost
Net::HTTP.start(uri.hostname, uri.port, use_ssl: true) { |http| http.request(req) }

Expected Behavior

When a baggage header already exists on the outgoing request, Sentry should merge its entries with the existing value by joining them with a comma (,), as per the W3C Bagga
ge specification. The resulting header should look like:

sentry-trace_id=abc123,sentry-environment=production,routingKey=myvalue,tenantId=123

Actual Behavior

Sentry replaces the entire baggage header with only Sentry's entries. The resulting header contains:

sentry-trace_id=abc123,sentry-environment=production,sentry-release=xyz,sentry-public_key=key123

The original routingKey=myvalue,tenantId=123 entries are lost.

Ruby Version

All (not version-specific)

SDK Version

All current versions (verified on 6.5.0)

Integration and Its Version

No response

Sentry Config

No response