Add Monotonic Types

[MrAlias]

Add the following types:

• MONOTONIC_INT64
• MONOTONIC_DOUBLE

These type values are still transported in the same DataPoints as their non-monotonic counterpoints, but add the significance to their values as being monotonic.

This is a partial replacement of #137
Relates to #125

[tigrannajaryan]

With 3 different enums we now use 3x32=96bits to encode 6 bits worth of info. This can have significant impact on memory consumption in Collector which may hold many thousands or even millions of metrics in queues.

Should we use a single field with bitmasks instead? We can keep the enums and just use a single uint32 with lots of reserved fields for future use.

[bogdandrutu]

@tigrannajaryan I see that as an optimization, which I think we can do it as a next step after we finalize the data format and all the properties we need.

[tigrannajaryan]

@MrAlias @bogdandrutu can we move forward with this? We are waiting for the finalization of metrics ProtoBufs to be able to use them in the Collector.

[MrAlias]

@MrAlias @bogdandrutu can we move forward with this? We are waiting for the finalization of metrics ProtoBufs to be able to use them in the Collector.

Sorry, was blocked working on open-telemetry/opentelemetry-collector-contrib#229 for a few days, but will get back to this today.

Talking with @bogdandrutu last week it sounds like two things (related to this PR) should be done:

• The protobuf should describe data post aggregation (instead of pre-instrument). This means the blocking conversation could be resolved by keeping things as is, but ...
• The concept of Monotonic doesn't apply to all datapoints (SummaryDataPoints), therefore, we should move the concept directly to the Type itself.

Going to update later to remove the Monotonic field and change Type to:

// Type is the type of values a metric has.
enum Type {
  INVALID_TYPE = 0;

  INT64 = 1;
  MONOTONIC_INT64 = 2;

  DOUBLE = 3;
  MONOTONIC_DOUBLE = 4;

  HISTOGRAM = 5;
  SUMMARY = 6;
}

There was an idea to include a MONOTONIC_HISTOGRAM as well, but I'm not sure how useful it will be. I'm open to opinions on this.

[MrAlias]

Looks like @jmacd has also been thinking about this: #125 (comment). Going to wait until after the Metric SIG meeting today to make any changes to make sure we are all in sync.

[jmacd]

I think we found resolution today in the SIG call.

[bogdandrutu]

@jmacd can you document the proposed resolution?

[MrAlias]

@jmacd can you document the proposed resolution?

@bogdandrutu : The plan is to go with:

// Type is the type of values a metric has.
enum Type {
  INVALID_TYPE = 0;

  INT64 = 1;
  MONOTONIC_INT64 = 2;

  DOUBLE = 3;
  MONOTONIC_DOUBLE = 4;

  HISTOGRAM = 5;
  SUMMARY = 6;
}

thoughts?

[bogdandrutu]

Can you please update the description?