A decade is an interval of time. In English, you know someone for a period of time. The "now" and the present perfect tense in your sentence both absolutely correctly indicate that in this case the period of time began in the past and is continuing to the present moment.
Sriram and I have known each other for a decade now.
"Since" marks a specific event: for example, the first time you and Sriram met.
Sriram and I have known each other since we met a decade ago.
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