Suggestions: Metrics with trait suggestions

[bshaffer]

Suggestions for #509

Major changes:

• Consolidate all header methods to one getMetricHeader for simplicity, and remove applyHeaders
• If a metrics header is supplied in the metadata, it will be used instead of the default (rather than having multiple metrics headers)
• Add headers in FetchAuthTokenCache when a cached token is found (rather than in CredentialsLoader)

Minor changes

• Make $credType a constant and add getCredType method to each Credential (and to UpdateMetadataInterface)
• Make $metricsHeaderKey a constant and move it to UpdateMetadataInterface::METRIC_METADATA_KEY (to be consistent with AUTH_METADATA_KEY)
• Rename all $metricsHeaders parameters to $headers, as this is more accurate.
• Import MetricsTrait in UpdateMetadataTrait (since it depends on this)
• Make getVersion private

[bshaffer]

Closing because these suggestions have been implemented