Prometheus exporter: handle colliding metric attribute keys

[dashpole]

From https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/prometheus_and_openmetrics.md#metric-attributes

OpenTelemetry Metric Attributes MUST be converted to Prometheus labels. String Attribute values are converted directly to Metric Attributes, and non-string Attribute values MUST be converted to string attributes following the attribute specification. Prometheus metric label keys are required to match the following regex: a-zA-Z_*. Metrics from OpenTelemetry with unsupported Attribute names MUST replace invalid characters with the _ character. Multiple consecutive _ characters MUST be replaced with a single _ character. This may cause ambiguity in scenarios where multiple similar-named attributes share invalid characters at the same location. In such unlikely cases, if multiple key-value pairs are converted to have the same Prometheus key, the values MUST be concatenated together, separated by ;, and ordered by the lexicographical order of the original keys.

This implements the last part of the spec, where values for colliding attributes are concatenated.

Question: are attributes sorted before they are passed to the prometheus exporter?

[codecov]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
.../opentelemetry/exporter/prometheus/Serializer.java	100.00%

📢 Thoughts on this report? Let us know!.

[mateuszrzeszutek]

Question: are attributes sorted before they are passed to the prometheus exporter?

Yes:

opentelemetry-java/api/all/src/main/java/io/opentelemetry/api/internal/ImmutableKeyValuePairs.java

Lines 46 to 52 in 0e63447

	/**
	* Sorts and dedupes the key/value pairs in {@code data}. {@code null} values will be removed.
	* Keys will be compared with the given {@link Comparator}.
	*/
	protected ImmutableKeyValuePairs(Object[] data, Comparator<?> keyComparator) {
	this(sortAndFilter(data, keyComparator));
	}

Attributes are always sorted by the keys' string values

[jack-berg]

Question: are attributes sorted before they are passed to the prometheus exporter?

Yes:

Technically that's an implementation detail of a specific Attributes implementation. Its possible to have other Attributes implementations, like the AttributesMap version we use to apply limits as attributes are recorded in the trace and log signals.

[dashpole]

@jack-berg is there anything else needed here?

[dashpole]

Thanks @jack-berg! Your additional changes LGTM