fix cache lookups and name sensing for recorded metrics

[memory]

A fix for most of the issues noted in https://github.com/Stackdriver/stackdriver-prometheus-sidecar/issues/104

• update targets.targetMatch() to match labels correctly in the case of a recorded metric (thank you @stevenycchou)

• update metadata.Cache.Get() to treat metrics with names prefixed with the value of the --stackdriver.recorded-metric-prefix flag (default: recorded_) as recorded metrics and log an error if a colon is found in the metric name (stackdriver metric names cannot have those)

• update README with specific notes for forwarding recorded metrics

[memory]

n.b. the build failure appears to be unrelated to this PR as far as I can tell

[memory]

Superseded by https://github.com/Stackdriver/stackdriver-prometheus-sidecar/pull/266