Clarify that 64 bit integer numbers are decimal strings in OTLP/JSON

[tigrannajaryan]

Resolves open-telemetry/opentelemetry-proto#268

This behavior is already the documented behavior for JSON Protobuf,
see https://developers.google.com/protocol-buffers/docs/proto3#json:

JSON value will be a decimal string. Either numbers or strings are accepted.

[obecny]

In collector exporter the number will be sent as intValue for numbers between -2147483648 to 2147483647;
Other numbers will be sent as doubleValue
https://github.com/open-telemetry/opentelemetry-js/blob/main/packages/opentelemetry-exporter-collector/src/transform.ts#L93

When we send data as proto then the whole data is being converted using protobufjs package which reads the proto file definitions and converts it automatically.

[tigrannajaryan]

In collector exporter the number will be sent as intValue for numbers between -2147483648 to 2147483647;
Other numbers will be sent as doubleValue
https://github.com/open-telemetry/opentelemetry-js/blob/main/packages/opentelemetry-exporter-collector/src/transform.ts#L93

@obecny Neither int32 nor double are good enough for timestamps (which is one of the most common uses cases for 64 bit integers in proto). int32 does not have enough range, double does not have enough precision. I do not know what are the options to deal with 64 bit integer numbers in JS and I understand it is painful but unfortunately this is the current reality of the proto.

This PR does not change that at all. This is just a clarification of how 64 bit integer numbers (which we have plenty in OTLP) work in Protobuf JSON encoding.

[reyang]

LGTM.