Prometheus Instrumentation

While the http metrics and manager are going to be instrumented in FFTM in https://github.com/hyperledger/firefly-transaction-manager/issues/38, it will still be useful for evmconnect to create its own custom metrics.

See https://github.com/hyperledger/firefly/blob/0b6c289ffee87330f578796d5ece8268216e133f/internal/metrics/prometheus.go for the firefly implementation of http and custom metrics.

- One metric that could be useful, is a counter for each submitted transaction.

- Another metric that will be useful would be block indexing progress metrics to capture issues such as https://github.com/hyperledger/firefly-evmconnect/pull/157