Fix serialization of arrays in prometheus exporter

[trask]

The spec says:

For protocols that do not natively support non-string values, non-string values SHOULD be represented as JSON-encoded strings.

but it looks like the Prometheus exporter calls only Object.toString():

opentelemetry-java/exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/Serializer.java

Line 502 in 9ac678e

String stringValue = value.toString();

[fstab]

Just FYI: I started working on a PR for #5940, and that will replace the Serializer class. I'm confident I can get the PR done next week.

[fstab]

Quick update: If #6015 gets merged this code will be moved to Otel2PrometheusConverter.convertAttributes(). Still the same issue though, #6015 also calls value.toString().