internal/telemetry: delay the conversion of metrics
internal/telemetry: delay the conversion of metrics

convert to wire form only when we need to encode it

Change-Id: Ib36ee2fbef773241a30b9aa707ebf311119b8705
Reviewed-on: https://go-review.googlesource.com/c/tools/+/208658
Run-TryBot: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
Reviewed-by: Nathan Dias <a76ce77789f7c2fecda031127b98bdbeb857c2cf@orijtech.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools