[Metrics SDK] Inconsistent CMake target name for Prometheus exporter

[lalitb]

Prometheus exporter cmake target name is not consistent with other otel-cpp targets:

$ ls -ltr /usr/local/lib/libopentelemetry_* /usr/local/lib/libprometheus_*
-rw-r--r-- 1 root root     3300 Nov  3 10:31 /usr/local/lib/libopentelemetry_version.a
-rw-r--r-- 1 root root   144864 Nov  3 10:31 /usr/local/lib/libopentelemetry_common.a
-rw-r--r-- 1 root root  1910868 Nov  3 10:31 /usr/local/lib/libopentelemetry_http_client_curl.a
-rw-r--r-- 1 root root  2794762 Nov  3 10:31 /usr/local/lib/libopentelemetry_resources.a
-rw-r--r-- 1 root root  5777132 Nov  3 10:31 /usr/local/lib/libopentelemetry_trace.a
-rw-r--r-- 1 root root 19872902 Nov  3 10:31 /usr/local/lib/libopentelemetry_metrics.a
-rw-r--r-- 1 root root  2444236 Nov  3 10:31 /usr/local/lib/libopentelemetry_exporter_in_memory.a
-rw-r--r-- 1 root root  2887480 Nov  3 10:31 /usr/local/lib/libopentelemetry_exporter_ostream_span.a
-rw-r--r-- 1 root root  1559756 Nov  3 10:31 /usr/local/lib/libopentelemetry_exporter_ostream_metrics.a
-rw-r--r-- 1 root root  2175544 Nov 10 16:15 /usr/local/lib/libprometheus_exporter.a
-rw-r--r-- 1 root root  4246384 Nov 10 16:17 /usr/local/lib/libopentelemetry_exporter_zipkin_trace.a