[TrackMetric] The `average` of the `trackMetric` call is sent as `value` which seems wrong

The current `trackMetric` API takes an object where the property `average` shows up in the portal as `valueSum` and the internal documentation states that it is the [value or average](https://github.com/microsoft/ApplicationInsights-JS/blob/3a1fdb18d7cd9536f11de91f61957f283c7811c5/shared/AppInsightsCommon/src/Interfaces/IMetricTelemetry.ts#L15-L19).

We need to
- investigate whether this can still be defined as the "average" and stored in the Portal as an "average" value
- If it's always a value or if "can" be either, then we "should" add another optional property (to keep backward compatibility) to provide `average` or `value` and if both are provided then it should "prefer" the `average` (unless the above investigation identifies the specific case / reason why it's either)

Adding and tagging as an enhancement to review as part of the backlog when we have cycles to investigate further.

Start with shared/AppInsightsCommon/src/Interfaces/IMetricTelemetry.ts and trace how trackMetric's average property appears in the portal as valueSum. Investigate whether the value is always an average or can represent either value, then define the backward-compatible API behavior and the conditions for considering the issue complete.

Written by the indexing model from the issue text.