[tracing] Link spans properly

[lmuhlha]

the http -> grpc spans are not linked.
the grpc -> grpc spans are not linked (cross region spans)
the http -> grpc -> bigtable spans are not linked correctly.

Any leanings on how to pass context between highly async requests should be documented for others.

[stale]

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[dmichel1]

After #615 the only thing left to do is link the http call to CoreQueryManager

[dmichel1]

This has been completed as of #621