Support old-style TensorFlow events (tensorboard)

[garymm]

Fixes: #2466

[google-oss-prow]

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[garymm]

I didn't see a place to add tests for this code. Is there a good place to add tests? If so, I'm happy to do it.

Also unrelated to this bug, but while testing I found it pretty surprising that you consider an event to match as long as the tag name starts with the metric name (e.g. a TF Event with tag "foobar" will match a metric name "foo"). Is this intended? Seems sort of surprising. If that's not intended, I can open another PR to fix.