Augment keys of all metric types in the same way

[stefanbirkner]

It is unexpected that the common key prefix for some metrics types is
different. This makes it unnecessary difficult to find the metrics in
the database. This commit unifies the prefix for all metrics.

The prefix of each key is now service name, type prefix, hostname. All
of them are optional. service name is added if it is available and not
set as a label because the configuration flag EnableServiceLabel is
set. type prefix is added if it is enabled with the configuration flag
EnableTypePrefix. hostname is added if it is available and not set
as a label because EnableHostnameLabel is set.

The behaviour of EmitKeys, IncrCounter, IncrCounterWithLabels,
AddSample, AddSampleWithLabels, MeasureSince and
MeasureSinceWithLabels changes with this commit. For EmitKey the
hostname will be added (details see above) but the service name will no
longer be added when EnableServiceLabel is set to true. For the
other types the hostname will be added if it is available and
EnableHostnameLabel is set to false.

[stefanbirkner]

This PR contains the changes of #20 and #76.

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