Tracing: Add enum for MessageEvent Type, Link Type and Span Kind

[mayurkale22]

Consider replacing string type with strict enum type for MessageEvent, Link and SpanKind (similar to other libraries). Specs: https://github.com/census-instrumentation/opencensus-proto/blob/6d79e46232e026136ae013489ef611ca721cb839/src/opencensus/proto/trace/v1/trace.proto#L186-L193

Example:

SpanKind

export enum SpanKind {
  UNSPECIFIED = 0,
  SERVER = 1,
  CLIENT = 2,
}

LinkType

export enum LinkType {
  UNSPECIFIED = 0,
  CHILD_LINKED_SPAN = 1,
  PARENT_LINKED_SPAN = 2,
}

MessageEventType

export enum MessageEventType {
  UNSPECIFIED = 0,
  SENT = 1,
  RECEIVED = 2,
}