Support dynamic stream metadata

[Pessimistress]

Change List

• Add getStreamMetadata to XVIZLoaderInterface
• Build local stream metadata if DYNAMIC_STREAM_METADATA config is set (requires Support dynamic stream metadata xviz#493)
• Switch core components to use getStreamMetadata instead of getMetadata().streams

[marionleborgne]

Thank you @Pessimistress 🙏 Just a few suggestions on:

• disambiguating between streamMetadata VS streamsMetadata
• updating the streamsMetadata refresh logic in XVIZLoaderInterface#getStreamsMetadata()

And a question in metrics-helper.js about a logical change.

[marionleborgne]

the second part of the if statement might be too lax (this.streamBuffer.streamCount > oldStreamCount). We could have the same number of streams, but different stream names. And in this case, we would fail to update streamsMetadata.

E.g:

{
   `/vehicle/pose`: ...,
   `/lidar`: ...
}

{
   `/vehicle/speed`: ...,
   `/lidar`: ...
}

I can see how doing a stream by stream comparison can be expensive one very update, so maybe we want to check on the exact streamNames list only periodically (every 1s?). Just an idea to not check this on every update.

[Pessimistress]

streamCount is an accumulative number since the creation of the buffer, and it only ever increases.

[coveralls]

Coverage increased (+0.2%) to 39.643% when pulling 5e1f00a on x/no-metadata into 0f0edff on master.