IndexError raised when parsing AWS `X-Amzn-Trace-Id` header

[korneliacieslik]

When the AWS X-Amzn-Trace-Id header is being parsed, the code assumes the presence of at least two - separators inside the Root= field. If the header is malformed or missing expected parts, this results in:

IndexError
index 1 outside of array bounds: -1...1 (IndexError)

This occurs at:

prometheus_exporter/lib/prometheus_exporter/middleware.rb

Lines 127 to 130 in 15e9d47

	# get the content of the x-amzn-trace-id header
	# see also: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-request-tracing.html
	value = env["HTTP_X_AMZN_TRACE_ID"]
	value&.split("Root=")&.last&.split("-")&.fetch(1)&.to_i(16)

This exception breaks metric collection and causes a 500 response

Would it be possible for the parser to return nil (or otherwise skip parsing) when the header is not a valid AWS trace header?