Prometheus exporter: Handle attribute key collisions from sanitization

[dashpole]

Is your feature request related to a problem?

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_]([a-zA-Z0-9_])*. 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.

Describe the solution you'd like

One solution is to group colliding keys, and then join them with ;, like the go exporter does:
https://github.com/open-telemetry/opentelemetry-go/blob/02616a25c68e674a04df01a1b7ba185a2db8cd5a/exporters/prometheus/exporter.go#L284-L305

Another solution is to sort the keys first, and then check whether it collides with the key from the previous iteration, like i'm working on in java: open-telemetry/opentelemetry-java#5717